Feature: attachment preview - reliable MIME + inline PDF on desktop and mobile

- MIME: Stalwart's download endpoint often returns application/octet-stream, so
  blob: previews silently downloaded (UUID filename) instead of rendering.
  Resolve the most specific MIME (attachment type -> filename ext -> blob type)
  and re-wrap the blob; also fixes inline preview for images and video.
- Desktop PDF: render via <iframe> (reliable for blob: PDFs) instead of <object>.
- Mobile PDF: no usable inline viewer (Android shows a blank frame / silent
  download; iOS Safari renders only the first page of a PDF in an <iframe>), so
  render with pdf.js (canvas, dynamic-imported so it stays off the desktop
  bundle; iOS-safe canvas cap). Double-tap zoom (fit -> 2x -> 3x -> fit) and
  2-finger pinch zoom (to 4x), both centred on the gesture and pannable via
  native scrolling.
- Route to pdf.js when navigator.pdfViewerEnabled is false (Android) and on iOS
  (incl. iPadOS, which reports true yet shows only the first page in a frame).
- Modal header gains an open-in-new-tab icon (next to download/close); the
  Android/browser Back button closes the preview instead of navigating the page.
- On a pdf.js render failure, offer an open-in-new-tab action as fallback.

locales/fr/common.json

@@ -2814,6 +2814,7 @@
     "hint": "Cliquez sur un e-mail à gauche pour commencer, ou lancez la visite."
   },
   "files": {
+    "open_in_new_tab": "Ouvrir dans un nouvel onglet",
     "title": "Fichiers",
     "search_placeholder": "Rechercher des fichiers...",
     "empty_state_title": "Aucun fichier pour le moment",