feat(metadata): add provider connection testing and token validation

Add UI and backend support for testing metadata provider credentials
before saving. Include parsing and validation for Bearer tokens,
display test results in the settings panel, and execute a test
connection specifically against the Hardcover provider.

Closes #127

packages/types/src/metadata-preferences.ts

@@ -127,3 +127,12 @@ export interface ProviderStatus {
   enabled: boolean;
   hint?: string;
 }
+
+export type ProviderConnectionTestStatus = "success" | "warning" | "fail";
+
+export interface ProviderConnectionTestResult {
+  key: MetadataProviderKey;
+  ok: boolean;
+  status: ProviderConnectionTestStatus;
+  message: string;
+}