feat: auto-detect login text color based on background image brightness

docs/admin-guide.md

@@ -235,7 +235,7 @@ Navigate to **Admin Settings** (owner only) to configure:
 
 ### Login Page Customization
 
-- **Background Image** — Upload a custom background image for the login page (max 5 MB, JPEG/PNG/WebP). The image is centered and fills the screen without distortion (`background-size: cover`).
+- **Background Image** — Upload a custom background image for the login page (max 5 MB, JPEG/PNG/WebP). The image is centered and fills the screen without distortion (`background-size: cover`). The text color (heading, subtitle, footer) is automatically adjusted based on the image brightness — light text for dark images, dark text for bright images.
 - **Card Style** — Choose between **Default** (solid card) and **Glass** (transparent, blurred backdrop).
 - **Subtitle** — Custom text shown below the application name on the login page. Leave empty for the default subtitle.
 

docs/environment-variables.md

@@ -121,4 +121,5 @@ In addition to environment variables, the following settings are configured thro
 | `lockout_duration` | `15` | Lockout duration in minutes |
 | `mfa_required` | `false` | Enforce MFA for all users |
 | `login_card_style` | `default` | Login card style: `default` or `glass` |
+| `login_text_color` | `light` | Login text color over background image: `light` or `dark` (auto-detected on upload) |
 | `login_subtitle` | _(empty)_ | Custom subtitle on the login page |