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/stories/archive/20_start_new_session.md

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+# Story 20: Start New Session / Clear Chat History
+
+## User Story
+As a user, I want to be able to start a fresh conversation without restarting the entire application, so that I can begin a new task with completely clean context (both frontend and backend) while keeping the same project open.
+
+## Acceptance Criteria
+- [x] There is a visible "New Session" or "Clear Chat" button in the UI
+- [x] Clicking the button clears all messages from the chat history (frontend)
+- [x] The backend conversation context is also cleared (no message history retained)
+- [x] The input field remains enabled and ready for a new message
+- [x] The button asks for confirmation before clearing (to prevent accidental data loss)
+- [x] After clearing, the chat shows an empty state or welcome message
+- [x] The project path and model settings are preserved (only messages are cleared)
+- [x] Any ongoing streaming or tool execution is cancelled before clearing
+- [x] The action is immediate and provides visual feedback
+
+## Out of Scope
+- Saving/exporting previous sessions before clearing
+- Multiple concurrent chat sessions or tabs
+- Undo functionality after clearing
+- Automatic session management or limits
+- Session history or recovery
+
+## Technical Notes
+- Frontend state (`messages` and `streamingContent`) needs to be cleared
+- Backend conversation history must be cleared (no retained context from previous messages)
+- Backend may need a `clear_session` or `reset_context` command
+- Cancel any in-flight operations before clearing
+- Should integrate with the cancellation mechanism from Story 13 (if implemented)
+- Button should be placed in the header area near the model selector
+- Consider using a modal dialog for confirmation
+- State: `setMessages([])` to clear the frontend array
+- Backend: Clear the message history that gets sent to the LLM
+
+## Design Considerations
+- Button placement: Header area (top right or near model controls)
+- Button style: Secondary/subtle to avoid accidental clicks
+- Confirmation dialog: "Are you sure? This will clear all messages and reset the conversation context."
+- Icon suggestion: 🔄 or "New" text label