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/features/auth/components/auth-layout.tsx

@@ -1,6 +1,7 @@
 import React from "react";
-import { Group, Text } from "@mantine/core";
+import { Group } from "@mantine/core";
 import classes from "./auth.module.css";
+import { BrandLogo } from "@/components/ui/brand-logo";
 
 type AuthLayoutProps = {
   children: React.ReactNode;
@@ -10,15 +11,7 @@ export function AuthLayout({ children }: AuthLayoutProps) {
   return (
     <>
       <Group justify="center" gap={8} className={classes.logo}>
-        <img
-          src="/icons/favicon-32x32.png"
-          alt="Gitmost"
-          width={22}
-          height={22}
-        />
-        <Text size="28px" fw={700} style={{ userSelect: "none" }}>
-          Gitmost
-        </Text>
+        <BrandLogo height={40} />
       </Group>
       <main>{children}</main>
     </>