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/files.json

@@ -55,7 +55,8 @@
     "keep": "Keep",
     "unkeep": "Stop keeping",
     "details": "Details…",
-    "upload": "Upload"
+    "upload": "Upload",
+    "view": "Preview"
   },
   "folder": {
     "menu": "Folder actions",
@@ -91,5 +92,15 @@
     "label": "A link to download",
     "placeholder": "Paste a link and I'll download it (YouTube, TikTok, Facebook…)",
     "download": "Download"
+  },
+  "preview": {
+    "prev": "Previous file",
+    "next": "Next file",
+    "position": "{{n}} / {{total}}",
+    "openInTab": "Open in a new tab",
+    "none": "There is no preview for this file",
+    "asText": "Shown as plain text",
+    "truncated": "Showing the start of this {{size}} file",
+    "wrap": "Wrap long lines"
   }
 }