fix(security): remove plaintext credential storage fallback

Credential writes now require Electron safeStorage encryption. Legacy plaintext values remain readable for migration and are encrypted during the next successful save. The UI override and status IPC are removed.

main.js

@@ -7,7 +7,6 @@ nativeTheme.themeSource = 'dark';
 const path = require('path');
 const fs = require('fs');
 const ConfigStore = require('./lib/config-store');
-const secretStore = require('./lib/secret-store');
 const UploadManager = require('./lib/upload-manager');
 const { createSourceFileCleanup } = require('./lib/source-file-cleanup');
 const SourceDeleteJournal = require('./lib/source-delete-journal');
@@ -1699,8 +1698,6 @@ ipcMain.handle('get-config', () => {
   return configStore.load();
 });
 
-ipcMain.handle('secret-store:status', () => ({ status: secretStore.getAvailabilityStatus() }));
-
 ipcMain.handle('save-config', async (_event, config) => {
   assertConfigWriteAllowed();
   await configStore.save(config);