feat(login): configurable logo size + hideable heading/subtitle

Login header customization for white-label deployments, all defaults
preserve current behaviour:

- LOGIN_LOGO_MAX_HEIGHT / LOGIN_LOGO_MAX_WIDTH (any CSS length): the logo
  box is otherwise a fixed 64x64 (w-16/h-16), which fits a wide wordmark to
  ~13px tall. When either is set, the fixed box is dropped and the logo
  renders at the configured size.
- LOGIN_SHOW_HEADING / LOGIN_SHOW_SUBTITLE (default true): hide the
  {appName} heading and/or the subtitle when the logo already reads as the
  brand (e.g. a wordmark) and they'd be redundant.

Applied to the standard login header; wired through the existing config
registry (CONFIG_ENV_MAP) -> /api/config -> useConfig.

Refs #519.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/api/config/route.ts

@@ -74,6 +74,10 @@ export async function GET(request: NextRequest) {
       loginImprintUrl: branded<string>('loginImprintUrl', ''),
       loginPrivacyPolicyUrl: branded<string>('loginPrivacyPolicyUrl', ''),
       loginWebsiteUrl: branded<string>('loginWebsiteUrl', ''),
+      loginLogoMaxHeight: configManager.get<string>('loginLogoMaxHeight', ''),
+      loginLogoMaxWidth: configManager.get<string>('loginLogoMaxWidth', ''),
+      loginShowHeading: configManager.get<boolean>('loginShowHeading', true),
+      loginShowSubtitle: configManager.get<boolean>('loginShowSubtitle', true),
       demoMode: configManager.get<boolean>('demoMode', false),
       allowCustomJmapEndpoint: configManager.get<boolean>('allowCustomJmapEndpoint', false),
       jmapServers: redactJmapServers(parseJmapServers(configManager.get<unknown>('jmapServers', []))),