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>

hooks/use-config.ts

@@ -26,6 +26,10 @@ interface ConfigData {
   loginImprintUrl: string;
   loginPrivacyPolicyUrl: string;
   loginWebsiteUrl: string;
+  loginLogoMaxHeight: string;
+  loginLogoMaxWidth: string;
+  loginShowHeading: boolean;
+  loginShowSubtitle: boolean;
   demoMode: boolean;
   autoSsoEnabled: boolean;
   allowCustomJmapEndpoint: boolean;
@@ -105,6 +109,10 @@ export function useConfig(): AppConfig {
     loginImprintUrl: configCache?.loginImprintUrl || '',
     loginPrivacyPolicyUrl: configCache?.loginPrivacyPolicyUrl || '',
     loginWebsiteUrl: configCache?.loginWebsiteUrl || '',
+    loginLogoMaxHeight: configCache?.loginLogoMaxHeight || '',
+    loginLogoMaxWidth: configCache?.loginLogoMaxWidth || '',
+    loginShowHeading: configCache?.loginShowHeading ?? true,
+    loginShowSubtitle: configCache?.loginShowSubtitle ?? true,
     demoMode: configCache?.demoMode || false,
     autoSsoEnabled: configCache?.autoSsoEnabled || false,
     allowCustomJmapEndpoint: configCache?.allowCustomJmapEndpoint || false,
@@ -140,6 +148,10 @@ export function useConfig(): AppConfig {
         loginImprintUrl: configCache.loginImprintUrl,
         loginPrivacyPolicyUrl: configCache.loginPrivacyPolicyUrl,
         loginWebsiteUrl: configCache.loginWebsiteUrl,
+        loginLogoMaxHeight: configCache.loginLogoMaxHeight,
+        loginLogoMaxWidth: configCache.loginLogoMaxWidth,
+        loginShowHeading: configCache.loginShowHeading,
+        loginShowSubtitle: configCache.loginShowSubtitle,
         demoMode: configCache.demoMode,
         autoSsoEnabled: configCache.autoSsoEnabled,
         allowCustomJmapEndpoint: configCache.allowCustomJmapEndpoint,
@@ -176,6 +188,10 @@ export function useConfig(): AppConfig {
           loginImprintUrl: data.loginImprintUrl,
           loginPrivacyPolicyUrl: data.loginPrivacyPolicyUrl,
           loginWebsiteUrl: data.loginWebsiteUrl,
+          loginLogoMaxHeight: data.loginLogoMaxHeight,
+          loginLogoMaxWidth: data.loginLogoMaxWidth,
+          loginShowHeading: data.loginShowHeading,
+          loginShowSubtitle: data.loginShowSubtitle,
           demoMode: data.demoMode,
           autoSsoEnabled: data.autoSsoEnabled,
           allowCustomJmapEndpoint: data.allowCustomJmapEndpoint,