feat: add unified right-click context menus across all components

[Mr54233]

Summary

Extracts a shared ContextMenu component (Radix UI) and adds right-click context menus to all major interactive areas of the app.

Closes #234

What's included

Shared component

• src/components/ContextMenu.tsx — generic context menu with typed MenuItem array, async onSelect with error catching, variant support (default/success/destructive)

Context menus added (8 components)

Component	Menu items
TaskListItem	Star, Rename, Resume, Mark done, Cancel, Reconnect, Merge worktree, Discard worktree, Delete (conditional on task status)
ProjectRail (vertical)	Open in file manager, Copy path, Hide/show, Delete
ProjectRail (expanded)	Same as above
FileExplorer / TreeItem	Replaces dedicated component with shared ContextMenu
FileViewer tab	Close, Close others, Close all, Copy path
GitChanges	Discard, Open file, Copy path
GitHistory	Copy commit hash
RunningView terminal	Copy, Paste, Clear, Select all + Cancel/Reconnect (status-dependent)
SessionView	Copy message

Bug fixes / refactors along the way

• Fix ProjectDrawer closing when clicking context menu items (Radix portal renders outside drawer ref)
• Remove fake MouseEvent hack in GitChanges onDiscard
• Async onSelect error catching in ContextMenu

Test plan

• Right-click each component listed above and verify menu appears
• Verify status-conditional items (e.g., detached tasks show "Reconnect", active tasks show "Cancel")
• Verify ProjectDrawer doesn't close when clicking context menu items inside it
• Verify GitChanges discard works without console errors