feat: network monitor, rate limiting, profile stats & UI/perf fixes

[Piyush-Thakkarr]

Summary

• NetworkMonitor: New NWPathMonitor-based utility with offline banner in MainAppView
• Gemini rate limiting: 2-second cooldown between API requests with user-facing error
• Profile stats: Total notes and total words count in ProfileView via SwiftData
• Email validation: @ and . check on sign-in and sign-up flows
• Adaptive sidebar: CodeWorkspace sidebar uses minWidth:160, idealWidth:200, maxWidth:260
• AI HTML wrapping: AI responses wrapped in <p> tags before injecting into RichTextEditor
• RichTextEditor optimization: lastSetHTML cache skips redundant HTML→attributed conversions
• filteredNotes memoization: Key-based cache avoids recomputing on every SwiftUI render
• iPad multi-window fix: Share sheet uses foregroundActive scene + isKeyWindow
• FormattingToolbar feedback: Haptic warning when applying format with no text selected
• Keyboard shortcut: Cmd+S save in NoteEditorView
• SwiftData migration: Added migration strategy comment for future schema changes

Test plan

• Build succeeds (verified)
• All 48 tests pass (verified)
• Toggle airplane mode → verify offline banner appears/disappears
• Send 2 rapid AI requests → verify rate limit message shows
• Open Profile → verify note count and word count display
• Try signing in with invalid email → verify validation error
• Rotate iPad / use split view → verify sidebar adapts and share sheet doesn't crash
• Use AI Replace/Append → verify HTML renders correctly in editor