Attachment previews, forwarded attachments, row dividers

Attachments in the reading pane are tiles with a preview: images and a PDF's
first page, rendered on the server (Pillow, pypdfium2) as WebP of at most
480 px, cached beside the blob store and in the browser. Tiles are all one
size — at most 230 px wide, narrower so three still fit in a row — and frame
each preview by its shape: close to the tile's it fills it, very tall / wide /
square ones show whole over a blurred copy, small ones are not enlarged, PDFs
show the top of the page. The name bar (checkbox, type, name, size, download)
slides in on hover and adapts to the tile's width; files without a preview
get a large type icon. Previews of opened conversations are kept offline.

Forward now carries the original's attachments: new compose attachments
sharing the stored blobs (POST /messages/{id}/forward-attachments).

Mailbox display / Settings → Appearance: "Show a dividing line between
rows" (off by default, per device), with a new switch. Settings drop the
"On a phone…" note, and "Language" reads "Spellchecking language".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/public/sw.js

@@ -51,7 +51,9 @@ const META = new Set([
 ]);
 const ID = "[0-9a-fA-F-]{36}";
 const THREAD_RE = new RegExp(`^/api/threads/(${ID})$`);
-const MESSAGE_RE = new RegExp(`^/api/messages/(${ID})/(?:html|attachments|cid/.+)$`);
+const MESSAGE_RE = new RegExp(
+  `^/api/messages/(${ID})/(?:html|attachments|attachments/${ID}/thumb|cid/.+)$`,
+);
 const ID_IN_PATH = new RegExp(ID);
 const PLUGIN_UI_RE = /^\/api\/plugins\/[^/]+\/ui\//; // plugin bundles (Labels' sidebar…)