fix(theming): use IAppConfig instead of deprecated IConfig::getAppValue

Replace two deprecated IConfig::getAppValue() calls in ThemingController
with IAppConfig::getAppValueString():

- getImage(): MIME type lookup for the original extensionless stored file
- getManifest(): cachebuster value lookup

Signed-off-by: Anna Larch <anna@nextcloud.com>
AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

apps/theming/lib/Controller/ThemingController.php

@@ -376,7 +376,7 @@ public function getImage(string $key, bool $useSvg = true) {
 		// application/octet-stream for it. Use the config-stored MIME type for the original
 		// file, and getMimeType() only for converted files which have a proper extension.
 		$mimeType = $file->getName() === $key
-			? $this->config->getAppValue($this->appName, $key . 'Mime', '')
+			? $this->appConfig->getAppValueString($key . 'Mime', '')
 			: $file->getMimeType();
 		$response->addHeader('Content-Type', $mimeType);
 		$response->addHeader('Content-Disposition', 'attachment; filename="' . $key . '"');
@@ -456,7 +456,7 @@ public function getThemeStylesheet(string $themeId, bool $plain = false, bool $w
 	#[BruteForceProtection(action: 'manifest')]
 	#[OpenAPI(scope: OpenAPI::SCOPE_DEFAULT)]
 	public function getManifest(string $app): JSONResponse {
-		$cacheBusterValue = $this->config->getAppValue('theming', 'cachebuster', '0');
+		$cacheBusterValue = $this->appConfig->getAppValueString('cachebuster', '0');
 		if ($app === 'core' || $app === 'settings') {
 			$name = $this->themingDefaults->getName();
 			$shortName = $this->themingDefaults->getName();

apps/theming/tests/Controller/ThemingControllerTest.php

@@ -676,10 +676,10 @@ public function testGetLogoOriginalFile(): void {
 		$this->imageManager->expects($this->once())
 			->method('getImage')
 			->willReturn($file);
-		$this->config
-			->expects($this->any())
-			->method('getAppValue')
-			->with('theming', 'logoMime', '')
+		$this->appConfig
+			->expects($this->once())
+			->method('getAppValueString')
+			->with('logoMime', '')
 			->willReturn('image/png');
 
 		@$expected = new FileDisplayResponse($file);
@@ -734,10 +734,10 @@ public static function dataGetManifest(): array {
 
 	#[\PHPUnit\Framework\Attributes\DataProvider(methodName: 'dataGetManifest')]
 	public function testGetManifest(bool $standalone): void {
-		$this->config
+		$this->appConfig
 			->expects($this->once())
-			->method('getAppValue')
-			->with('theming', 'cachebuster', '0')
+			->method('getAppValueString')
+			->with('cachebuster', '0')
 			->willReturn('0');
 		$this->themingDefaults
 			->expects($this->any())