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/app.py

@@ -33,6 +33,13 @@ def create_app() -> Quart:
         level=getattr(logging, Config.LOG_LEVEL.upper(), logging.INFO),
     )
 
+    # Validate config early so misconfigurations surface immediately with clear messages
+    try:
+        Config.validate()
+    except ValueError as exc:
+        logging.getLogger(__name__).error("Invalid configuration:\n%s", exc)
+        raise
+
     app = Quart(__name__, static_folder=None)
     app.secret_key = Config.SECRET_KEY
     app.config["SESSION_COOKIE_HTTPONLY"] = True