feat: add project context UI in settings panel

[nonogv]

Summary

• Adds WebSocket message types and state for per-project context (ProjectState, ContextState)
• Extends Settings panel with Project, Instructions, and Memories sections (global + per-project layers)
• Adds ProjectStaleBanner amber warning bar with "Update Memories" and dismiss actions

What was NOT touched

• src/storage.ts, src/server.ts, src/agent/chat.ts (backend PRs handle server contract)

Tradeoffs

• Settings textareas use local draft state synced from server context on prop change, so unsaved edits are preserved until the server pushes new context
• context_saved dismisses the stale banner locally; full memory refresh requires the backend PR

Test plan

• npm run build passes
• Open settings → switch/create project, verify slug display and Clear
• Edit global/project instructions and memories; project fields disabled without active slug
• Stale banner appears on project_stale message; Update Memories sends refresh_project_memories

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