feat: add configurable logo to login page with light/dark mode support

- Display Bulwark logo on the login page, with theme-aware switching
  between light and dark variants based on the resolved theme
- Add LOGIN_LOGO_LIGHT_URL and LOGIN_LOGO_DARK_URL env variables to
  allow custom logo overrides (defaults to bundled Bulwark branding)
- Expose logo config through /api/config endpoint and useConfig hook
- Add logo URL prompts to the setup.sh installer branding step
- Rename branding assets to use underscores instead of spaces

.env.example

@@ -76,8 +76,16 @@ JMAP_SERVER_URL=https://your-jmap-server.com
 # Login Page Customization (all optional)
 # =============================================================================
 
+# Custom logo images for the login page (PNG, SVG, etc.)
+# Can be absolute URLs or paths relative to the public/ directory.
+# Light mode logo (shown on light backgrounds), defaults to Bulwark logo.
+LOGIN_LOGO_LIGHT_URL=/branding/Bulwark_Logo_Color.svg
+#
+# Dark mode logo (shown on dark backgrounds), defaults to Bulwark white logo.
+LOGIN_LOGO_DARK_URL=/branding/Bulwark_Logo_Color.svg
+
 # Company or organization name displayed above the version on the login page
-# LOGIN_COMPANY_NAME=My Company
+LOGIN_COMPANY_NAME=Bulwark Webmail
 
 # URL for the imprint/legal notice link on the login page
 # LOGIN_IMPRINT_URL=https://example.com/imprint
@@ -86,7 +94,7 @@ JMAP_SERVER_URL=https://your-jmap-server.com
 # LOGIN_PRIVACY_POLICY_URL=https://example.com/privacy
 
 # URL for the company website link on the login page
-# LOGIN_WEBSITE_URL=https://example.com
+LOGIN_WEBSITE_URL=https://bulwarkmail.org
 
 # =============================================================================
 # Legacy Build-time Variables (still supported as fallback)