feat(ui): natural markdown rendering in editor blocks & agent panel

- Editor: applyAIBlocks() and fdApplyDocument(replace) now convert agent
  markdown into real blocks via md2b() (## -> heading_2, GFM tables ->
  table block, ` -> code, lists, to-do, quote, divider). No more literal
  '## ' shown.
- Editor: inline markdown rendered in blocks via mdEsc() (bold/italic/
  inline-code/links/strikethrough) in renderBlock for paragraphs, headings,
  lists, to-do, toggle, quote, callout, code.
- Agent panel: renderMarkdown() renders assistant responses as HTML (H1-H4,
  GFM tables, code blocks, lists+checkboxes, quotes, hr, inline marks);
  content escaped before rendering (XSS-safe vs LLM). CSS .fd-md-* added.
- VERSION + app.main -> 5.1.1; CHANGELOG updated.
- 271 tests pass (no regression); markdown render verified in Node; editor &
  panel render 200 with new components.

app/main.py

@@ -60,7 +60,7 @@ async def lifespan(_app: FastAPI):
 
 app = FastAPI(
     title="FlowDeck",
-    version="5.1.0",
+    version="5.1.1",
     docs_url="/docs" if settings.log_level == "DEBUG" else None,
     redoc_url=None,
     lifespan=lifespan,