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.

docs/branding/assets/logo-on-light.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 196 48" width="196" height="48" role="img" aria-label="Archivr">
+  <defs>
+    <style>@import url('https://fonts.googleapis.com/css2?family=Cormorant+Garamond:wght@600&amp;display=swap');</style>
+  </defs>
+  <!-- Mark -->
+  <rect x="4" y="4" width="40" height="40" rx="6.4" fill="#8d3f30"/>
+  <text x="24" y="24" text-anchor="middle" dominant-baseline="central"
+        dx="0.02em" dy="0.03em"
+        font-family="'Cormorant Garamond', Georgia, serif"
+        font-weight="600" font-size="30" fill="#f4ead8">A</text>
+  <!-- Wordmark -->
+  <text x="58" y="24" text-anchor="start" dominant-baseline="central"
+        font-family="'Cormorant Garamond', Georgia, serif"
+        font-weight="600" font-size="28" letter-spacing="0.28" fill="#20251f">archivr</text>
+</svg>