feat: UI Integration Iteration 4 - Profile/Statistics Screens (#33)

* feat: UI Integration Iteration 4 - Profile/Statistics Screens

Implements Profile and Statistics screens, completing Iteration 4 of UI Integration phase.

## Changes

**New Files**:
- src/ui/render/profile.rs (225 lines) - Profile screen with level, XP bar, stats, quests
- src/ui/render/statistics.rs (238 lines) - Statistics screen with performance breakdown

**Modified Files**:
- src/ui/render/mod.rs - Added new modules and ViewMode dispatch
- src/main.rs - Enhanced key handling for navigation (p, s, m keys)
- Cargo.lock - Updated dependencies

## Features

**Profile Screen**:
- Level display with star emoji
- XP progress bar using Gauge widget
- Statistics: total scenarios, perfect runs, total XP
- Daily quests: active count, completed today
- Navigation: 's' for statistics, 'm' for menu

**Statistics Screen**:
- Performance breakdown with horizontal bars (Perfect/Excellent/Good/Fair/Poor)
- Quest statistics: completed count, current streak, longest streak
- Session history: average score, total time played
- Navigation: 'p' for profile, 'm' for menu

**Navigation Flow**:
- Results → Profile (press 'p')
- Profile ⇄ Statistics (press 's'/'p')
- Any screen → Menu (press 'm')

## Testing

- ✅ 308/308 tests passing (100%)
- ✅ Zero clippy warnings (strict mode)
- ✅ Clean release build (27.74s)
- ✅ Follows Elm architecture pattern

## Code Quality

- Modular design with separate render modules
- RefCell safety with scoped borrows
- Doc comments for all public functions
- Consistent styling and color schemes

* fix: Address critical issues from agent review

Fixes two critical issues identified during comprehensive agent review
before merging Phase 1 UI Integration.

## Issues Fixed

### 1. XP Overflow Protection (Critical)
**File**: src/gamification/profile.rs:100
**Issue**: Used unchecked addition which could overflow and panic
**Fix**: Replace `+=` with `saturating_add()`

**Before**:
```rust
self.total_xp += xp;  // Could overflow u64 and panic
```

**After**:
```rust
self.total_xp = self.total_xp.saturating_add(xp);
```

**Risk**: Very low (would take 21 billion years at 1 scenario/sec to overflow),
but better to be safe and prevent any possibility of panic.

### 2. Event Loop Error Handling (High Priority)
**File**: src/main.rs:246
**Issue**: Used `.unwrap()` in event loop without explanation
**Fix**: Replace with `.expect()` with clear message

**Before**:
```rust
let digit = c.to_digit(10).unwrap() as usize;
```

**After**:
```rust
let digit = c
    .to_digit(10)
    .expect("char is validated as ascii_digit by guard condition")
    as usize;
```

**Safety**: Guard condition `c.is_ascii_digit()` ensures this never panics,
but explicit expect makes intent clear and aids debugging.

## Verification

- ✅ All 308 tests passing
- ✅ Zero clippy warnings
- ✅ Code formatted with nightly rustfmt
- ✅ Clean release build

## Agent Review Results

- ✅ Performance: APPROVED (5ms render, 69% headroom)
- ✅ Security: APPROVED (0 unsafe, 0 CVEs, proper RefCell usage)
- 🟡 Testing: CONDITIONAL GO (75% coverage, these fixes address critical issues)
- ✅ Code Review: GO (excellent architecture, Rust best practices)

Author: bug-ops

Cargo.lock

@@ -97,7 +97,7 @@ impl UserProfile {
     /// ```
     pub fn add_xp(&mut self, xp: u64) -> bool {
         let old_level = self.level;
-        self.total_xp += xp;
+        self.total_xp = self.total_xp.saturating_add(xp);
         self.level = XPCalculator::level_from_xp(self.total_xp);
         self.level > old_level
     }

src/gamification/profile.rs

@@ -219,14 +219,17 @@ fn handle_key_event(key: KeyEvent, state: &AppState) -> Option<Message> {
         ui::Screen::MainMenu => handle_menu_keys(key, state),
         ui::Screen::Task => handle_task_keys(key, state),
         ui::Screen::Results => handle_results_keys(key),
-        ui::Screen::Profile | ui::Screen::Statistics => handle_profile_stats_keys(key),
+        ui::Screen::Profile | ui::Screen::Statistics => handle_profile_stats_keys(key, state),
     }
 }
 
 /// Handle keyboard events on profile and statistics screens
-fn handle_profile_stats_keys(key: KeyEvent) -> Option<Message> {
+fn handle_profile_stats_keys(key: KeyEvent, state: &AppState) -> Option<Message> {
     match key.code {
-        KeyCode::Esc | KeyCode::Char('q') => Some(Message::BackToMenu),
+        KeyCode::Esc | KeyCode::Char('m') => Some(Message::BackToMenu),
+        KeyCode::Char('q') => Some(Message::QuitApp),
+        KeyCode::Char('s') if state.screen == ui::Screen::Profile => Some(Message::ShowStatistics),
+        KeyCode::Char('p') if state.screen == ui::Screen::Statistics => Some(Message::ShowProfile),
         _ => None,
     }
 }
@@ -240,7 +243,10 @@ fn handle_menu_keys(key: KeyEvent, state: &AppState) -> Option<Message> {
         KeyCode::Enter => Some(Message::MenuSelect),
         // Quick jump with number keys (1-9)
         KeyCode::Char(c) if c.is_ascii_digit() => {
-            let digit = c.to_digit(10).unwrap() as usize;
+            let digit = c
+                .to_digit(10)
+                .expect("char is validated as ascii_digit by guard condition")
+                as usize;
             if digit >= 1 && digit <= state.scenarios.len() {
                 // Jump to scenario (digit - 1 because scenarios are 0-indexed)
                 Some(Message::StartScenario(digit - 1))
@@ -363,6 +369,7 @@ fn handle_results_keys(key: KeyEvent) -> Option<Message> {
         KeyCode::Char('q') => Some(Message::QuitApp),
         KeyCode::Char('r') => Some(Message::RetryScenario),
         KeyCode::Char('m') => Some(Message::BackToMenu),
+        KeyCode::Char('p') => Some(Message::ShowProfile),
         _ => None,
     }
 }

src/main.rs

@@ -8,7 +8,9 @@ mod editor;
 mod helpers;
 mod menu;
 mod popups;
+mod profile;
 mod results;
+mod statistics;
 mod task;
 
 #[cfg(test)]
@@ -31,53 +33,7 @@ pub fn render(frame: &mut Frame, state: &mut AppState) {
         Screen::MainMenu => menu::render_main_menu(frame, state),
         Screen::Task => task::render_task_screen(frame, state),
         Screen::Results => results::render_results_screen(frame, state),
-        Screen::Profile => render_profile_screen_placeholder(frame, state),
-        Screen::Statistics => render_statistics_screen_placeholder(frame, state),
+        Screen::Profile => profile::render_profile_screen(frame, state),
+        Screen::Statistics => statistics::render_statistics_screen(frame, state),
     }
 }
-
-/// Placeholder for profile screen
-// TODO: Iteration 4 - Implement full profile screen with achievements, stats, level progress
-fn render_profile_screen_placeholder(frame: &mut Frame, _state: &mut AppState) {
-    use ratatui::{
-        layout::Alignment,
-        style::{Color, Modifier, Style},
-        widgets::{Block, Borders, Paragraph},
-    };
-
-    let area = frame.area();
-    let placeholder =
-        Paragraph::new("Profile Screen - Coming Soon!\n\nPress Esc to return to menu")
-            .style(
-                Style::default()
-                    .fg(Color::Cyan)
-                    .add_modifier(Modifier::BOLD),
-            )
-            .alignment(Alignment::Center)
-            .block(Block::default().title(" Profile ").borders(Borders::ALL));
-
-    frame.render_widget(placeholder, area);
-}
-
-/// Placeholder for statistics screen
-// TODO: Iteration 4 - Implement statistics with mastery breakdown, activity graph, weak commands
-fn render_statistics_screen_placeholder(frame: &mut Frame, _state: &mut AppState) {
-    use ratatui::{
-        layout::Alignment,
-        style::{Color, Modifier, Style},
-        widgets::{Block, Borders, Paragraph},
-    };
-
-    let area = frame.area();
-    let placeholder =
-        Paragraph::new("Statistics Screen - Coming Soon!\n\nPress Esc to return to menu")
-            .style(
-                Style::default()
-                    .fg(Color::Cyan)
-                    .add_modifier(Modifier::BOLD),
-            )
-            .alignment(Alignment::Center)
-            .block(Block::default().title(" Statistics ").borders(Borders::ALL));
-
-    frame.render_widget(placeholder, area);
-}