feat: CalDAV credential persistence in Secret Service

Move CalDAV username/password from in-memory config to Secret Service
(gnome-keyring). CalDAVSyncProvider now loads credentials at sync time
and erases them on disconnect. OAuthSecretStore extended with
store/load/erase_caldav_credential methods using a separate libsecret
schema.

tests/microsoft_sync_provider_tests.cpp

@@ -40,6 +40,7 @@ using nocal::sync::HttpTransport;
 using nocal::sync::OAuthClientConfig;
 using nocal::sync::OAuthSecretStore;
 using nocal::sync::OAuthTokens;
+using nocal::sync::CalDAVCredential;
 using nocal::sync::SyncObserver;
 using nocal::sync::SyncProgressEvent;
 using nocal::sync::SyncStage;
@@ -223,6 +224,18 @@ public:
         ++erase_calls;
         store_data.erase(account_id);
     }
+
+    void store_caldav_credential(std::string, const CalDAVCredential&,
+        std::stop_token = {}) override {
+        std::terminate();
+    }
+    [[nodiscard]] std::optional<CalDAVCredential> load_caldav_credential(
+        std::string, std::stop_token = {}) override {
+        std::terminate();
+    }
+    void erase_caldav_credential(std::string, std::stop_token = {}) override {
+        std::terminate();
+    }
 };
 
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