fix(tui): hydrate lazy startup panel and use animated loaders

The lazy startup panel could remain stuck on the placeholder when no first
prompt was submitted because agent construction only started from _sess(). Keep
session.create cheap, but schedule _start_agent_build shortly after returning
the placeholder so tools/skills hydrate automatically.

Also replace the ugly placeholder bar rows with compact unicode-animations
braille loaders for the tools and skills sections.

Tests:
- python -m py_compile tui_gateway/server.py
- cd ui-tui && npm run type-check && npm run build
- cd ui-tui && npm test -- --run src/__tests__/useSessionLifecycle.test.ts src/__tests__/useConfigSync.test.ts
- scripts/run_tests.sh tests/tui_gateway/test_protocol.py::test_sess_found tests/tools/test_code_execution_modes.py tests/tools/test_code_execution.py

tui_gateway/server.py

@@ -1726,6 +1726,17 @@ def _(rid, params: dict) -> dict:
         "transport": current_transport() or _stdio_transport,
     }
 
+    # Return the lightweight session immediately so Ink can paint the composer
+    # + skeleton panel, then build the real AIAgent just after this response is
+    # flushed.  This keeps startup responsive while still hydrating tools/skills
+    # without requiring the user to submit a first prompt.
+    def _deferred_build() -> None:
+        session = _sessions.get(sid)
+        if session is not None:
+            _start_agent_build(sid, session)
+
+    threading.Timer(0.05, _deferred_build).start()
+
     return _ok(
         rid,
         {