frontend: redesign the thread reading panel

Rework the conversation view to match the new dark-theme mockups:
- Subject bar gains a print button and an expand/collapse-all toggle (default
  action expands all; collapsing leaves only the newest message open).
- Collapsed messages are compact single-line cards (40px avatar, colored sender
  name, snippet, relative date) that overlap into a stack; alignment follows the
  sort order (top edge for oldest-first, bottom edge for newest-first) and the
  always-open newest card overlaps its neighbour in both orders.
- Expanded messages show sender+address (whole header row toggles collapse), a
  detail disclosure (From/To/Cc/Subject/Date/Size), the message location
  (Inbox/Sent), a read/unread circle (per-message flag toggle), a quick
  Reply/Reply-all, and an Actions menu (reply/forward/print/delete). The
  Reply/Reply all/Forward bar now sits inside the newest card above a divider.
- Message body: the quoted original is hidden by default with a "•••" toggle
  injected after the "On … wrote:" attribution; the iframe now collapses-then-
  measures so it shrinks back when the quote is hidden.
- Avatars use the sender's IMAP-account colour when the address matches one of
  our accounts, else a stable per-sender hue; two initials for multi-word names.
- Brighter dark-theme border token; new print/expand-all/collapse-all icons.

Backend: add size_bytes to the message envelope (shown in message details).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/i18n/en.ts

@@ -53,6 +53,8 @@ export const en = {
   "mail.selectConversation": "Select a conversation.",
   "mail.loadingThread": "Loading thread…",
   "mail.expandAll": "Expand all",
+  "mail.collapseAll": "Collapse all",
+  "mail.print": "Print",
   "mail.orderOldest": "Oldest first",
   "mail.orderNewest": "Newest first",
   "filter.all": "All mail",
@@ -79,6 +81,15 @@ export const en = {
   "mail.loadImages": "Load images",
   "mail.alwaysFromSender": "Always from this sender",
   "mail.to": "to",
+  "mail.from": "From",
+  "mail.subjectLabel": "Subject",
+  "mail.dateLabel": "Date",
+  "mail.sizeLabel": "Size",
+  "mail.actions": "Actions",
+  "mail.forwardAsAttachment": "Forward as attachment",
+  "mail.showQuoted": "Show quoted text",
+  "mail.hideQuoted": "Hide quoted text",
+  "mail.markReadUnread": "Mark read / unread",
   "mail.messageCount": "{count} messages",
   "mail.downloadAll": "Download all {count} attachments",
   "mail.downloadZip": "Download zip",