feat(branding): dark-mode logo variant

Add an optional dark-mode logo (branding_logo_url_dark) alongside the
main logo. Upload/remove via the logo endpoint (?variant=dark) on the
Branding settings page. Admin header (admin dark mode) and the public
gallery (dark themes) pick the dark logo when active, falling back to
the light logo when unset. PDFs keep using the light logo.

backend/src/routes/adminSettings.js

@@ -544,9 +544,16 @@ router.post('/logo', adminAuth, requirePermission('settings.edit'), upload.singl
       return res.status(400).json({ error: 'No logo file uploaded' });
     }
 
+    // ?variant=dark stores a separate dark-mode logo (branding_logo_*_dark);
+    // anything else is the default (light) logo. Consumers pick the dark
+    // variant when the active theme is dark, falling back to the light one.
+    const isDark = req.query.variant === 'dark' || req.body.variant === 'dark';
+    const pathKey = isDark ? 'branding_logo_path_dark' : 'branding_logo_path';
+    const urlKey = isDark ? 'branding_logo_url_dark' : 'branding_logo_url';
+
     // Get old logo to delete
     const oldLogoSetting = await db('app_settings')
-      .where('setting_key', 'branding_logo_path')
+      .where('setting_key', pathKey)
       .first();
 
     if (oldLogoSetting && oldLogoSetting.setting_value) {
@@ -568,7 +575,7 @@ router.post('/logo', adminAuth, requirePermission('settings.edit'), upload.singl
 
     await db('app_settings')
       .insert({
-        setting_key: 'branding_logo_path',
+        setting_key: pathKey,
         setting_value: JSON.stringify(logoPath),
         setting_type: 'branding',
         updated_at: new Date()
@@ -582,7 +589,7 @@ router.post('/logo', adminAuth, requirePermission('settings.edit'), upload.singl
     // Save public URL
     await db('app_settings')
       .insert({
-        setting_key: 'branding_logo_url',
+        setting_key: urlKey,
         setting_value: JSON.stringify(publicPath),
         setting_type: 'branding',
         updated_at: new Date()
@@ -603,6 +610,36 @@ router.post('/logo', adminAuth, requirePermission('settings.edit'), upload.singl
   }
 });
 
+// Remove a logo. ?variant=dark clears the dark-mode logo
+// (branding_logo_*_dark); otherwise the default logo. Best-effort file
+// unlink, then blanks the url + path settings.
+router.delete('/logo', adminAuth, requirePermission('settings.edit'), async (req, res) => {
+  try {
+    const isDark = req.query.variant === 'dark';
+    const pathKey = isDark ? 'branding_logo_path_dark' : 'branding_logo_path';
+    const urlKey = isDark ? 'branding_logo_url_dark' : 'branding_logo_url';
+
+    const pathSetting = await db('app_settings').where('setting_key', pathKey).first();
+    if (pathSetting && pathSetting.setting_value) {
+      try {
+        let p = pathSetting.setting_value;
+        if (p.startsWith('"')) p = JSON.parse(p);
+        await fs.unlink(p);
+      } catch (error) {
+        console.error('Failed to delete logo file:', error);
+      }
+    }
+    await db('app_settings')
+      .whereIn('setting_key', [pathKey, urlKey])
+      .update({ setting_value: JSON.stringify(''), updated_at: new Date() });
+
+    res.json({ message: 'Logo removed' });
+  } catch (error) {
+    console.error('Logo delete error:', error);
+    res.status(500).json({ error: 'Failed to remove logo' });
+  }
+});
+
 // Upload watermark logo
 router.post('/branding/watermark-logo', adminAuth, requirePermission('settings.edit'), upload.single('watermarkLogo'), async (req, res) => {
   try {

backend/src/routes/publicSettings.js

@@ -59,6 +59,7 @@ router.get('/', async (req, res) => {
       branding_watermark_size: settingsObject.branding_watermark_size || 15,
       branding_favicon_url: settingsObject.branding_favicon_url || '',
       branding_logo_url: settingsObject.branding_logo_url || '',
+      branding_logo_url_dark: settingsObject.branding_logo_url_dark || '',
       branding_logo_size: settingsObject.branding_logo_size || 'medium',
       branding_logo_max_height: settingsObject.branding_logo_max_height || 48,
       branding_logo_position: settingsObject.branding_logo_position || 'left',