feat(brand): roll out Gitmost logo, favicon and app name

Replace the bare brand text on pages with the Gitmost logo lockup
(mark + "gitmost" wordmark) and use the mark as the favicon.

- add generated logo lockups (text outlined from Space Grotesk SemiBold)
  in dark/light ink variants; add reusable theme-aware <BrandLogo> component
- use BrandLogo in the global header (mark-only on mobile, full lockup on
  desktop) and on auth pages, dropping the old Docmost icon + plain text
- point favicon to /brand/gitmost-favicon.svg (SVG primary + PNG fallbacks);
  regenerate favicon/app-icon PNGs from the brand SVGs
- rename app name Docmost -> Gitmost (getAppName, index.html title/apple
  title, manifest name); use getAppName() in the 404 title
- align theme/background colors to the brand tile (#0E1117)
- move brand guide and logos into docs/brand/ (canonical home) with a README,
  and serve runtime copies from apps/client/public/brand/

apps/client/public/manifest.json

@@ -1,10 +1,10 @@
 {
-  "name": "Docmost",
-  "short_name": "Docmost",
+  "name": "Gitmost",
+  "short_name": "Gitmost",
   "start_url": "/",
   "display": "standalone",
-  "background_color": "#222",
-  "theme_color": "#222",
+  "background_color": "#0E1117",
+  "theme_color": "#0E1117",
   "icons": [
     {
       "src": "icons/favicon-16x16.png",