fix(sync): show context-aware permission error for Flatpak/Snap

Add isFlatpak() detection to Electron API and show environment-specific
error messages when file permission errors occur during sync:
- Flatpak users see Flatseal and ~/.var/app/ guidance
- Snap users see 'snap connect' and ~/snap/... guidance
- Other users see generic permission error

Addresses #4078

electron/electronAPI.d.ts

@@ -83,6 +83,8 @@ export interface ElectronAPI {
 
   isSnap(): boolean;
 
+  isFlatpak(): boolean;
+
   // SEND
   // ----
   reloadMainWin(): void;

electron/preload.ts

@@ -59,6 +59,7 @@ const ea: ElectronAPI = {
   isLinux: () => process.platform === 'linux',
   isMacOS: () => process.platform === 'darwin',
   isSnap: () => process && process.env && !!process.env.SNAP,
+  isFlatpak: () => process && process.env && !!process.env.FLATPAK_ID,
 
   // SEND
   // ----

src/app/imex/sync/sync-wrapper.service.ts

@@ -35,6 +35,7 @@ import { DialogIncoherentTimestampsErrorComponent } from './dialog-incoherent-ti
 import { SyncLog } from '../../core/log';
 import { promiseTimeout } from '../../util/promise-timeout';
 import { devError } from '../../util/dev-error';
+import { IS_ELECTRON } from '../../app.constants';
 
 @Injectable({
   providedIn: 'root',
@@ -244,7 +245,7 @@ export class SyncWrapperService {
         return 'HANDLED_ERROR';
       } else if (this._isPermissionError(error)) {
         this._snackService.open({
-          msg: T.F.SYNC.S.ERROR_PERMISSION,
+          msg: this._getPermissionErrorMessage(),
           type: 'ERROR',
           config: { duration: 12000 },
         });
@@ -388,6 +389,16 @@ export class SyncWrapperService {
     return /EROFS|EACCES|EPERM|read-only file system|permission denied/i.test(errStr);
   }
 
+  private _getPermissionErrorMessage(): string {
+    if (IS_ELECTRON && window.ea?.isFlatpak?.()) {
+      return T.F.SYNC.S.ERROR_PERMISSION_FLATPAK;
+    }
+    if (IS_ELECTRON && window.ea?.isSnap?.()) {
+      return T.F.SYNC.S.ERROR_PERMISSION_SNAP;
+    }
+    return T.F.SYNC.S.ERROR_PERMISSION;
+  }
+
   private lastConflictDialog?: MatDialogRef<any, any>;
 
   private _openConflictDialog$(

src/app/t.const.ts

@@ -1179,6 +1179,8 @@ const T = {
         ERROR_DATA_IS_CURRENTLY_WRITTEN: 'F.SYNC.S.ERROR_DATA_IS_CURRENTLY_WRITTEN',
         ERROR_FALLBACK_TO_BACKUP: 'F.SYNC.S.ERROR_FALLBACK_TO_BACKUP',
         ERROR_PERMISSION: 'F.SYNC.S.ERROR_PERMISSION',
+        ERROR_PERMISSION_FLATPAK: 'F.SYNC.S.ERROR_PERMISSION_FLATPAK',
+        ERROR_PERMISSION_SNAP: 'F.SYNC.S.ERROR_PERMISSION_SNAP',
         ERROR_INVALID_DATA: 'F.SYNC.S.ERROR_INVALID_DATA',
         ERROR_NO_REV: 'F.SYNC.S.ERROR_NO_REV',
         ERROR_UNABLE_TO_READ_REMOTE_DATA: 'F.SYNC.S.ERROR_UNABLE_TO_READ_REMOTE_DATA',

src/assets/i18n/en.json

@@ -1159,7 +1159,9 @@
         "BTN_FORCE_OVERWRITE": "Force Overwrite",
         "ERROR_CORS": "WebDAV Sync Error: Network request failed.\n\nThis might be a CORS issue. Please ensure:\n• Your WebDAV server allows Cross-Origin requests\n• The server URL is correct and accessible\n• You have a working internet connection",
         "ERROR_DATA_IS_CURRENTLY_WRITTEN": "Remote Data is currently being written",
-        "ERROR_PERMISSION": "File access denied. If using Flatpak/Snap, grant filesystem permission via Flatseal or use a path inside ~/.var/app/",
+        "ERROR_PERMISSION": "File access denied. Please check your filesystem permissions.",
+        "ERROR_PERMISSION_FLATPAK": "File access denied. Grant filesystem permission via Flatseal or use a path inside ~/.var/app/",
+        "ERROR_PERMISSION_SNAP": "File access denied. Run 'snap connect super-productivity:home' or use a path inside ~/snap/super-productivity/common/",
         "ERROR_FALLBACK_TO_BACKUP": "Something went wrong while importing the data. Falling back to local backup.",
         "ERROR_INVALID_DATA": "Error while syncing. Invalid data",
         "ERROR_NO_REV": "No valid rev for remote file",