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/src/i18n/en.ts

@@ -362,7 +362,7 @@ export const en = {
   "settings.spellcheck": "Spelling and grammar check",
   "settings.on": "On",
   "settings.off": "Off",
-  "settings.spellcheckLanguage": "Language",
+  "settings.spellcheckLanguage": "Spellchecking language",
   "settings.spellcheckAuto": "Auto-detect (each paragraph)",
   "settings.spellcheckUnavailable": "Not available: this server has no spell-checking engine set up.",
   "settings.recoveryEmailHint":
@@ -484,7 +484,7 @@ export const en = {
   "display.side": "Side",
   "display.bottom": "Bottom",
   "display.off": "Off",
-  "display.phoneNote": "On a phone, mail always opens in a single panel.",
+  "display.dividers": "Show a dividing line between rows",
   "admin.mail": "Outgoing system mail",
   "admin.mailHelp":
     "The mail account the server sends password-reset links from. Without it, “Forgot password?” sends nothing.",
@@ -561,6 +561,7 @@ export const en = {
   "offline.draftsNotSaved": "Only drafts written on this device are available offline.",
   "compose.sendLater": "Schedule",
   "compose.savedOnDevice": "Saved on this device",
+  "compose.forwardAttachFailed": "Couldn't attach the original message's files.",
   "compose.attachOnDevice": "On this device — uploaded when you're back online",
   "compose.onDevice": "On this device",
   "nav.scheduled": "Scheduled",