feat: add troubleshooting settings with open app data and reset workspace

Add a Troubleshooting section under a Support nav group in Settings:

- Open App Data Folder: reveals the user-data directory in the OS file
  explorer via shell.openPath.
- Reset Local Workspace: destructive action that deletes workspace.json
  and scratchpad contents, clears the in-memory cache, and reseeds
  defaults. Auth secrets are preserved. Requires two-step confirmation.

New IPC channels, ElectronApi methods, and preload bridge added for
openAppDataFolder and resetLocalWorkspace. Existing electron mock in
tests updated to include shell.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/channels.ts

@@ -27,6 +27,8 @@ export const ipcChannels = {
   selectProject: 'selection:project',
   selectPattern: 'selection:pattern',
   selectSession: 'selection:session',
+  openAppDataFolder: 'troubleshooting:open-app-data-folder',
+  resetLocalWorkspace: 'troubleshooting:reset-local-workspace',
   workspaceUpdated: 'workspace:updated',
   sessionEvent: 'sessions:event',
 } as const;