Project-aware assist, link suggestions, project-scoped RAG, semantic search tool, SSE race fix

- Writing assistant: inject project notes as context (definition-tagged first), wikilink suggestions
- Link suggestions: server-side endpoint finds unlinked term occurrences, NoteEditorView sidebar panel
- Project-scoped RAG: ChatView ProjectSelector filters semantic+keyword search to selected project
- Semantic search tool: LLM search_notes upgraded to hybrid semantic (0.40 threshold) + keyword merge
- SSE race condition fix: drain remaining events after stream loop exits in chat.py and notes.py
- RAG_AUTO_SNIPPET raised 800→4000; sidebar include uses full note body; MAX_BODY_CHARS 8000→24000
- Enter-to-submit on writing assistant instruction textareas (note and task editors)
- DiffView: equal-line collapsing with 3-line context around changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/routes/chat.py

@@ -116,6 +116,7 @@ async def send_message_route(conv_id: int):
     include_note_ids = data.get("include_note_ids") or []
     excluded_note_ids = data.get("excluded_note_ids") or []
     think = bool(data.get("think", False))
+    rag_project_id = data.get("rag_project_id") or None
 
     # Reject if generation already running for this conversation
     existing = get_buffer(conv_id)
@@ -128,7 +129,10 @@ async def send_message_route(conv_id: int):
     # Create placeholder assistant message
     assistant_msg = await add_message(conv_id, "assistant", "", status="generating")
 
-    buf = create_buffer(conv_id, assistant_msg.id)
+    try:
+        buf = create_buffer(conv_id, assistant_msg.id)
+    except RuntimeError:
+        return jsonify({"error": "Generation already in progress"}), 409
 
     # Build history from existing messages (excluding system and the placeholder)
     history = []
@@ -146,6 +150,7 @@ async def send_message_route(conv_id: int):
         include_note_ids=include_note_ids,
         excluded_note_ids=excluded_note_ids,
         think=think,
+        rag_project_id=rag_project_id,
     ))
 
     return jsonify({
@@ -192,6 +197,11 @@ async def generation_stream_route(conv_id: int):
             if not got_new:
                 yield ": keepalive\n\n"
 
+        # Final drain: deliver any events appended between the last yield
+        # and the state check above (e.g. the 'done' event).
+        for event in buf.events_after(cursor):
+            yield buf.format_sse(event)
+
     return Response(
         stream(),
         content_type="text/event-stream",