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/src/components/ui/error-404.tsx

@@ -3,6 +3,7 @@ import classes from "./error-404.module.css";
 import { Link } from "react-router-dom";
 import { Helmet } from "react-helmet-async";
 import { useTranslation } from "react-i18next";
+import { getAppName } from "@/lib/config";
 
 export function Error404() {
   const { t } = useTranslation();
@@ -10,7 +11,7 @@ export function Error404() {
   return (
     <>
       <Helmet>
-        <title>{t("404 page not found")} - Docmost</title>
+        <title>{t("404 page not found")} - {getAppName()}</title>
       </Helmet>
       <Container className={classes.root}>
         <Title className={classes.title}>{t("404 page not found")}</Title>