chore: add brand identity, style guide, and favicon (#35)

* Add Archivr brand identity and favicon

- Brand style guide (docs/branding/style-guide.html): logo, clear
  space, don'ts, color palette, typography, in-context mockup
- Logo assets (docs/branding/assets/): lockup SVGs for dark/light
  backgrounds, standalone mark, favicon.svg, multi-size favicon.ico
- Wire favicon into frontend/index.html (SVG + ICO fallback);
  frontend/public/ seeded so Vite copies assets on next build
- Revert display strings archivr back to Archivr in UI
  (Topbar, LoginPage, SetupPage, index.html titles, flake.nix)
- Update .gitignore to allow docs/branding/

* Build frontend with favicon links and assets

* Fix static file serving: serve all of static/ with SPA index.html fallback

Previously only /assets/* was served from disk; all other paths
(including /favicon.ico, /favicon.svg) hit the index.html fallback.
Replace nest_service("/assets") + fallback_service(index.html) with
ServeDir on the full static dir and not_found_service(index.html),
so any file that exists in static/ is served directly.

crates/archivr-server/src/routes.rs

@@ -387,8 +387,7 @@ pub fn app_with_state(state: AppState) -> Router {
             get(blob_cleanup_scan_handler).delete(blob_cleanup_delete_handler),
         )
         .route("/api/util/resolve-tco", post(resolve_tco_handler))
-        .nest_service("/assets", ServeDir::new(static_dir.join("assets")))
-        .fallback_service(ServeFile::new(static_dir.join("index.html")))
+        .fallback_service(ServeDir::new(&static_dir).not_found_service(ServeFile::new(static_dir.join("index.html"))))
         .layer(axum::middleware::from_fn_with_state(
             state.clone(),
             setup_guard,