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/logo-mark-v0.svg

@@ -0,0 +1,20 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 120 120" width="120" height="120">
+  <defs>
+    <style>
+      @import url('https://fonts.googleapis.com/css2?family=Cormorant+Garamond:wght@600&amp;display=swap');
+      .mark-letter {
+        font-family: "Cormorant Garamond", Georgia, serif;
+        font-weight: 600;
+        font-size: 74px;
+        fill: #f4ead8;
+        dominant-baseline: middle;
+        text-anchor: middle;
+      }
+    </style>
+  </defs>
+
+  <!-- Accent background -->
+  <rect x="4" y="4" width="112" height="112" rx="16" ry="16" fill="#8d3f30"/>
+  <!-- Letter a -->
+  <text class="mark-letter" x="60" y="63">a</text>
+</svg>