refactor: clean up web assets and consolidate PHP helpers

- Remove 2079 unused bootstrap-icons SVGs + min.css (~9.3 MB saved, app uses font approach only)
- Remove unused editorjs.umd.js (only .mjs is imported)
- Extract shared syncControlState into app-conditional-controls.js, deduplicate SSO/LDAP toggles
- Add settingToBool() helper to DRY up repeated bool-parsing pattern across 6 call sites
- Merge single-function auth.php into app.php, delete auth.php
- Extract 9 branding functions from app.php into dedicated branding.php (app.php 709→528 lines)
- Remove empty web/img/ directory

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/Service/Settings/SettingsAppGateway.php

@@ -121,12 +121,10 @@ class SettingsAppGateway
 
     public function isUserThemeAllowed(): bool
     {
-        $value = $this->settingsMetadataGateway->getValue(SettingKeys::APP_THEME_USER_KEY);
-        if ($value === null || $value === '') {
-            return true;
-        }
-
-        return in_array(strtolower($value), ['1', 'true', 'yes', 'on'], true);
+        return settingToBool(
+            $this->settingsMetadataGateway->getValue(SettingKeys::APP_THEME_USER_KEY),
+            true
+        );
     }
 
     public function setUserThemeAllowed(bool $allowed, ?string $description = null): bool
@@ -138,12 +136,10 @@ class SettingsAppGateway
 
     public function isRegistrationEnabled(): bool
     {
-        $value = $this->settingsMetadataGateway->getValue(SettingKeys::APP_REGISTRATION_KEY);
-        if ($value === null || $value === '') {
-            return true;
-        }
-
-        return in_array(strtolower($value), ['1', 'true', 'yes', 'on'], true);
+        return settingToBool(
+            $this->settingsMetadataGateway->getValue(SettingKeys::APP_REGISTRATION_KEY),
+            true
+        );
     }
 
     public function setRegistrationEnabled(bool $allowed, ?string $description = null): bool