feat(editor): Notion AI-style block UX (hint, compact slash menu, inline AI composer)

- Empty paragraph placeholder: "Press space for AI or / for commands".
- Slash menu redesigned: compact items (icon + name + shortcut #/##/### on
  right), friendly group labels, 'Close menu (esc)' footer, 'Type to search'
  input at bottom, and hover preview card (e.g. 'Our Values' H2 sample).
- Inline AI composer (AIC): space on empty block opens 'Edit with AI' pill;
  Enter -> agent generate with animated 'Brewing...' (stop button) -> framed
  markdown result + thumbs/thumbdown + 'Insert below' -> applyAIBlocks (md2b).
- CSS: ai-pulse animation, .aici-result styles, .sm-* tweaks; app.css cache.
- VERSION + app.main -> 5.1.2; CHANGELOG updated.
- 271 tests pass (no regression); editor renders 200 (hint/menu/AIC present);
  JS validated via node --check.

app/main.py

@@ -60,7 +60,7 @@ async def lifespan(_app: FastAPI):
 
 app = FastAPI(
     title="FlowDeck",
-    version="5.1.1",
+    version="5.1.2",
     docs_url="/docs" if settings.log_level == "DEBUG" else None,
     redoc_url=None,
     lifespan=lifespan,