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/README.md

@@ -1,4 +1,4 @@
-# archivr
+# Archivr
 
 An open-source self-hosted archiving tool. Work in progress.
 
@@ -65,9 +65,9 @@ This project aims to provide a reliable solution for archiving important data fr
 - Direct platform URLs
 - Platform shorthand inputs such as `tweet:...`, `yt:...`, or `instagram:...`
 
-## Running Archivr
+## Running archivr
 
-Archivr currently ships as two binaries:
+archivr currently ships as two binaries:
 
 - `archivr`
   - The CLI for creating and writing to one archive.