Story 20: New Session button to clear chat history

- Added 'New Session' button in header with confirmation dialog
- Clears frontend messages, streaming content, and loading state
- Button has subtle styling with hover effects
- Preserves project path, model selection, and tool settings
- Backend context automatically cleared (no server-side history)
- Fixed build errors by removing unused tokio watch channel
- All quality checks passing (TypeScript, Biome, Clippy, builds)

Tested and verified:
- Button appears in header near model controls
- Confirmation dialog prevents accidental clearing
- Messages clear immediately after confirmation
- Input remains ready for new conversation
- Settings preserved across session clears

.living_spec/specs/functional/UI_UX.md

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+
+## Session Management
+
+### Problem
+Users may want to start a fresh conversation without restarting the application. Long conversations can become unwieldy, and users need a way to clear context for new tasks while keeping the same project open.
+
+### Solution: New Session Button
+Provide a clear, accessible way for users to start a new session by clearing the chat history.
+
+### Requirements
+
+1. **Button Placement:** Located in the header area, near model controls
+2. **Visual Design:** Secondary/subtle styling to prevent accidental clicks
+3. **Confirmation Dialog:** Ask "Are you sure? This will clear all messages." before clearing
+4. **State Management:**
+   - Clear `messages` state array
+   - Clear `streamingContent` if any streaming is in progress
+   - Preserve project path, model selection, and tool settings
+   - Cancel any in-flight backend operations before clearing
+5. **User Feedback:** Immediate visual response (messages disappear)
+6. **Empty State:** Show a welcome message or empty state after clearing
+
+### Implementation Notes
+
+**Frontend:**
+- Add "New Session" button to header
+- Implement confirmation modal/dialog
+- Call `setMessages([])` after confirmation
+- Cancel any ongoing streaming/tool execution
+- Consider keyboard shortcut (e.g., Cmd/Ctrl+K)
+
+**Backend:**
+- May need to cancel ongoing chat operations
+- Clear any server-side state if applicable
+- No persistent session history (sessions are ephemeral)
+
+**Edge Cases:**
+- Don't clear while actively streaming (cancel first, then clear)
+- Handle confirmation dismissal (do nothing)
+- Ensure button is always accessible (not disabled)
+
+### Button Label Options
+- "New Session" (clear and descriptive)
+- "Clear Chat" (direct but less friendly)
+- "Start Over" (conversational)
+- Icon: 🔄 or ⊕ (plus in circle)