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/config.py

@@ -73,3 +73,20 @@ class Config:
     @classmethod
     def searxng_enabled(cls) -> bool:
         return bool(cls.SEARXNG_URL)
+
+    @classmethod
+    def validate(cls) -> None:
+        """Validate critical config values at startup. Raises ValueError on misconfiguration."""
+        errors: list[str] = []
+        if cls.OLLAMA_NUM_CTX < 512:
+            errors.append(f"OLLAMA_NUM_CTX={cls.OLLAMA_NUM_CTX} is too small (minimum 512)")
+        if cls.LOG_RETENTION_DAYS < 1:
+            errors.append(f"LOG_RETENTION_DAYS={cls.LOG_RETENTION_DAYS} must be >= 1")
+        if cls.IMAGE_MAX_BYTES < 1024:
+            errors.append(f"IMAGE_MAX_BYTES={cls.IMAGE_MAX_BYTES} must be >= 1024")
+        if not (1 <= cls.SMTP_PORT <= 65535):
+            errors.append(f"SMTP_PORT={cls.SMTP_PORT} must be between 1 and 65535")
+        if cls.oidc_enabled() and not cls.BASE_URL.startswith(("http://", "https://")):
+            errors.append(f"BASE_URL='{cls.BASE_URL}' must start with http:// or https:// when OIDC is enabled")
+        if errors:
+            raise ValueError("Configuration errors:\n" + "\n".join(f"  - {e}" for e in errors))