fix: use local timezone for date-based features (check-in, usage stats)

Root cause: `new Date().toISOString().slice(0, 10)` returns UTC date,
causing date-based features to be off by up to ±1 day for non-UTC users.

Check-in / assistant workspace:
- Add `getLocalDateString()` utility, replace all UTC date comparisons
- Schema v2→v3 migration: normalize lastCheckInDate only when it matches
  UTC today (ambiguous window); leave past dates untouched so check-in
  triggers correctly after upgrade
- Runtime compat in `needsDailyCheckIn`: accept UTC today as fallback
  for edge cases the migration cannot catch
- `needsDailyCheckIn(state, now?)` accepts optional time for testability

Usage stats:
- Replace single-offset SQL bucketing (`DATE(datetime(..., '+N minutes'))`)
  with JS-side per-row aggregation using `getLocalDateString(new Date(utc))`
- Date methods account for historical DST offset at each message's timestamp
- `getTokenUsageStats(days, now?)` accepts optional time to pin window start
- Window boundary via `localDayStartAsUTC()` is already DST-aware

Tests (new file: timezone-boundaries.test.ts, 22 tests):
- getLocalDateString at UTC+9/UTC-5/UTC boundaries with fixed timestamps
- needsDailyCheckIn: compat suppression, clearly-past trigger, local match
- v2→v3 migration: targeted normalization, past-date preservation, null safe
- localDayStartAsUTC: UTC+8/UTC-5 midnight boundaries
- Usage stats bucketing: UTC+9 cross-day, summary window, DST spring-forward
- All usage stats tests use pinned `now` — no time-bomb from fixed test data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 7418

src/__tests__/unit/assistant-workspace.test.ts

@@ -16,6 +16,7 @@ import assert from 'node:assert/strict';
 import path from 'path';
 import os from 'os';
 import fs from 'fs';
+import { getLocalDateString } from '@/lib/utils';
 
 // Set a temp data dir before importing db module
 const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'codepilot-workspace-test-'));
@@ -59,7 +60,7 @@ describe('Assistant Workspace', () => {
       const state = JSON.parse(fs.readFileSync(statePath, 'utf-8'));
       assert.equal(state.onboardingComplete, false);
       assert.equal(state.lastCheckInDate, null);
-      assert.equal(state.schemaVersion, 2);
+      assert.equal(state.schemaVersion, 3);
     });
 
     it('should create all 4 template files', () => {
@@ -90,7 +91,7 @@ describe('Assistant Workspace', () => {
       initializeWorkspace(workDir);
       const state = loadState(workDir);
       state.onboardingComplete = true;
-      state.lastCheckInDate = new Date().toISOString().slice(0, 10);
+      state.lastCheckInDate = getLocalDateString();
       saveState(workDir, state);
 
       const reloaded = loadState(workDir);
@@ -133,13 +134,13 @@ describe('Assistant Workspace', () => {
     });
 
     it('should not trigger check-in if already done today', () => {
-      const today = new Date().toISOString().slice(0, 10);
+      const today = getLocalDateString();
       const state = { onboardingComplete: true, lastCheckInDate: today, schemaVersion: 2 };
       assert.equal(needsDailyCheckIn(state), false);
     });
 
     it('onboarding day should skip daily check-in (lastCheckInDate set)', () => {
-      const today = new Date().toISOString().slice(0, 10);
+      const today = getLocalDateString();
       const state = { onboardingComplete: true, lastCheckInDate: today, schemaVersion: 2 };
       assert.equal(needsDailyCheckIn(state), false);
     });
@@ -262,20 +263,20 @@ describe('Assistant Workspace', () => {
       migrateStateV1ToV2(workDir);
 
       const state = loadState(workDir);
-      assert.equal(state.schemaVersion, 2);
+      assert.equal(state.schemaVersion, 3);
       assert.ok(fs.existsSync(path.join(workDir, 'memory', 'daily')));
       assert.ok(fs.existsSync(path.join(workDir, 'Inbox')));
     });
 
-    it('should not re-migrate v2 state', () => {
+    it('should not re-migrate v3 state', () => {
       initializeWorkspace(workDir);
       const state = loadState(workDir);
-      assert.equal(state.schemaVersion, 2);
+      assert.equal(state.schemaVersion, 3);
 
       // Should not throw or change anything
       migrateStateV1ToV2(workDir);
       const reloaded = loadState(workDir);
-      assert.equal(reloaded.schemaVersion, 2);
+      assert.equal(reloaded.schemaVersion, 3);
     });
   });