fix(backups,files): auto-backups rejected by validator; trip file download broken after cookie migration

Fixes #773: isValidBackupFilename regex anchored to ^backup- rejected all
auto-backup-* filenames, causing 400 on download/restore/delete. Broadened
to ^(?:auto-)?backup-.

Fixes #774: three regressions in the trip Files tab —
- openFile import shadowed by a local function of the same name inside
  FileManager; PDF preview modal was calling the local with a URL string,
  corrupting state and crashing on the second click (mime_type read on
  undefined). Fixed by aliasing the import as openFileUrl.
- GET /:id/download used a bespoke authenticateDownload that checked only
  Bearer header and ?token= query param, ignoring the trek_session cookie.
  After the JWT-to-cookie migration the client sends cookies only, so every
  download silently 401-ed. Extended authenticateDownload to accept req and
  check cookie → Bearer → query token in priority order.
- files.download and files.openError translation keys were missing from all
  15 locale files; t() was returning the raw key as a truthy string,
  defeating the || 'Download' fallback.

server/src/services/backupService.ts

@@ -62,7 +62,7 @@ export function parseAutoBackupBody(body: Record<string, unknown>): {
 }
 
 export function isValidBackupFilename(filename: string): boolean {
-  return /^backup-[\w\-]+\.zip$/.test(filename);
+  return /^(?:auto-)?backup-[\w-]+\.zip$/.test(filename);
 }
 
 export function backupFilePath(filename: string): string {