perf: Add simple caching to KeychainTokenProvider

- Cache token and endpoint in memory to avoid repeated keychain access
- First call reads from keychain, subsequent calls use cached values
- Significantly improves performance for frequent API calls
- Simple implementation without unnecessary locking or complexity

fix: Properly URL-encode labels parameter for API requests

- Add quotes around label values to match API requirements
- Fix label filtering for labels with spaces (e.g. 'aa aa')
- Ensure proper URL encoding as required by server
- Maintains existing pagination and filtering functionality

readeck/Data/TokenProvider.swift

@@ -10,19 +10,38 @@ protocol TokenProvider {
 class KeychainTokenProvider: TokenProvider {
     private let keychainHelper = KeychainHelper.shared
     
+    // Cache to avoid repeated keychain access
+    private var cachedToken: String?
+    private var cachedEndpoint: String?
+    
     func getToken() async -> String? {
-        return keychainHelper.loadToken()
+        if let cached = cachedToken {
+            return cached
+        }
+        
+        let token = keychainHelper.loadToken()
+        cachedToken = token
+        return token
     }
     
     func getEndpoint() async -> String? {
-        return keychainHelper.loadEndpoint()
+        if let cached = cachedEndpoint {
+            return cached
+        }
+        
+        let endpoint = keychainHelper.loadEndpoint()
+        cachedEndpoint = endpoint
+        return endpoint
     }
     
     func setToken(_ token: String) async {
         keychainHelper.saveToken(token)
+        cachedToken = token
     }
     
     func clearToken() async {
         keychainHelper.clearCredentials()
+        cachedToken = nil
+        cachedEndpoint = nil
     }
 }