feat(sessions): add ability to move chat sessions between worktrees

[BowgartField]

Summary

• Add "Move to Worktree" context menu option on session tabs (right-click) to move a session from one worktree to another within the same project
• When the source worktree has uncommitted changes, a confirmation dialog asks whether to migrate them (via git stash/pop) or move the session only
• Automatically navigates to the target worktree after a successful move
• Clears CLI session IDs on move (they're tied to the original working directory) and saves a conversation transcript as a per-session context file so Claude retains conversation history in the new worktree

Changes

Backend (Rust):

• storage.rs: New move_session_to_worktree() — atomically moves session between worktree indices with deadlock-safe locking, clears CLI session IDs, updates metadata
• commands.rs: New Tauri command with optional git stash/pop for uncommitted changes migration + conversation transcript generation
• lib.rs: Register command
• dispatch.rs: Web access dispatch match arm

Frontend (React):

• MoveSessionDialog.tsx: New AlertDialog with 3 options (move with changes / without / cancel)
• SessionChatModal.tsx: Full integration — fetches sibling worktrees, handles move logic, uncommitted changes detection, navigation after move
• SessionListRow.tsx: "Move to Worktree" submenu in context menu
• session-card-utils.tsx: New props for worktree list and move handler
• ProjectCanvasView.tsx: Wire up onNavigateToWorktree callback

Test plan

• Open a project with 2+ worktrees
• Right-click a session tab → "Move to Worktree" → pick a target
• On clean worktree: session moves immediately with toast
• On dirty worktree: dialog appears with 3 options
• After move: modal navigates to target worktree with moved session selected
• Send a message referencing prior conversation → Claude has context via transcript
• Move last session from a worktree → source shows empty session list