fix(editor): AI composer 400, slash-menu arrows, drag handle, block outline, hint

- AI (space) -> 400 DeepSeek: fdAgent.generate now uses the clean no-tools
  /api/agent/generate endpoint instead of the tool-enabled conversation engine
  (which sends tools/tool_choice schemas DeepSeek rejects). Fixes the composer,
  Ask AI, AI meeting note and summarize flows.
- Slash menu arrows: selection highlight was invisible on open (nothing marked),
  so up/down appeared dead. Call _rs() on open + after filtering, and give the
  selected item an accent background + left bar. Verified with jsdom harness:
  Down/Up move selection, Enter applies.
- Block outline when selected: suppress focus rings (outline/box-shadow) on all
  editable blocks ([data-bid], .block-content, page title).
- Drag handle (6-dot): add .block-wrapper:hover/:focus-within .block-handle so
  it shows on hover for normal blocks (only column-wrappers showed it before).
- Empty-block hint: brighter (--text-secondary, opacity .75) so the
  "Press space for AI or / for commands" text is clearly visible on empty paras.
- VERSION + app.main -> 5.1.3; CHANGELOG updated.
- 271 tests pass; page renders 200; /api/agent/generate route registered;
  JS validated (node --check + jsdom).

app/main.py

@@ -60,7 +60,7 @@ async def lifespan(_app: FastAPI):
 
 app = FastAPI(
     title="FlowDeck",
-    version="5.1.2",
+    version="5.1.3",
     docs_url="/docs" if settings.log_level == "DEBUG" else None,
     redoc_url=None,
     lifespan=lifespan,