feat(files): a file that is not media opens in a viewer — an image, a PDF, text as text

- GET /api/files/{id}/preview: an allow-list on the stored extension (downloads/preview.py) picks
  image / PDF / text; served inline under `CSP: sandbox; frame-ancestors 'self'`, the type from the
  table; HTML and SVG arrive as plain text; a text is its first 1 MiB (X-Preview-Truncated), sent
  as UTF-8 whatever it was written in (BOM, UTF-8, else cp1250); a NUL means binary: 415
- the list says which preview each file gets (asset.preview); FE/BE contract on kinds and header
- the viewer (shared dialog shell): ← / → through the list's such files in its order, Save, open in
  a tab, the file's menu; one stage height for every kind; a no-preview card offers the file
- the name, the card's picture and the menu's Preview open it; an image up to 8 MB is its own
  thumbnail

frontend/src/i18n/locales/en/errors.json

@@ -237,6 +237,7 @@
     "file_folder_name_taken": "You already have a folder with that name",
     "file_folder_not_own": "Only a folder you made can be renamed or deleted",
     "file_folder_gone": "That folder no longer exists",
-    "file_folder_not_its_source": "A file can only go into one of your own folders or back to the folder it came from"
+    "file_folder_not_its_source": "A file can only go into one of your own folders or back to the folder it came from",
+    "file_no_preview": "There is no preview for this file"
   }
 }