feat: loading-screen status overlay (no more silent 'Comfy' splash)

Adds a small status line on ComfyUI's loading splash showing whether
Tenaciousload is serving object_info from cache or building it (live node
count + elapsed time, with a 'scanning model folders over the network' hint
when the CIFS walk stalls progress). Removes itself on the app 'setup' hook.

Backend tracks build progress (_build_state) and exposes GET /tenaciousload/status;
frontend web/loading-status.js polls it and renders an unobtrusive overlay.
Unit-tested progress tracking + status shape.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -103,6 +103,10 @@ python main.py --listen --port 8188 --enable-compress-response-body
   negligible compared to the bytes saved over the network.
 
 ## Notes
+- **Loading status:** instead of ComfyUI's silent "Comfy" splash, a small status
+  line shows whether it's *serving from cache* or *building* (with node count +
+  elapsed time), so a long rebuild isn't a black screen with no feedback. It
+  removes itself once the app is ready. Status is also at `GET /tenaciousload/status`.
 - The disk cache lives in `./cache/` (git-ignored). Delete it, or use the refresh
   button, to force a rebuild.
 - An nginx reverse proxy can cache `object_info` at the HTTP layer too, but this