Two-way draft sync, signatures, sent/bcc fixes, and mail-list UI

Backend
- Sent copies: persist locally on send + periodic multi-folder CONDSTORE poll
  (Sent/Archive/Trash/Junk) so they thread and show without an IDLE-only INBOX.
- Bcc: transmitted copy omits Bcc, Sent copy keeps it; store/display bcc_addr.
- Drafts: mirror local drafts to the IMAP Drafts folder (append/replace/expunge);
  inbound sync of foreign drafts (un-threaded) with adopt-to-edit + dedup by UID.
- Per-user storage quota (5 GiB default) and manual "sync all folders" action.
- Account editing (display/color/server/password) and per-account HTML signature.
- Account view scoped to that account's inbox; new migrations 0006–0009.

Frontend
- Mail-list pagination, folder filters (All/Unread/Pinned), sort toggle, refresh.
- Aligned + widened mail-list margins; reading-pane bars fixed height.
- Rich-text composer: color palette, highlight, image; attachments; expand/pop-out.
- Signatures: auto-insert for new mail, replace on From change, reply insert.
- Restore/Delete actions per folder; drafts view merges local + remote, date-sorted.

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

frontend/src/i18n/en.ts

@@ -10,6 +10,7 @@ export const en = {
   "common.saving": "Saving…",
   "common.delete": "Delete",
   "common.remove": "Remove",
+  "common.edit": "Edit",
   "common.close": "Close",
   "common.back": "Back",
   "common.next": "Next",
@@ -45,6 +46,8 @@ export const en = {
   "nav.spam": "Spam",
   "nav.trash": "Trash",
   "nav.search": "Search mail…",
+  "nav.storageQuota": "Storage used",
+  "nav.refresh": "Refresh all folders",
 
   "mail.noConversations": "No conversations.",
   "mail.selectConversation": "Select a conversation.",
@@ -57,6 +60,8 @@ export const en = {
   "filter.pinned": "Pinned",
   "filter.sortOldest": "Sort oldest first",
   "filter.sortNewest": "Sort newest first",
+  "pager.prev": "Previous page",
+  "pager.next": "Next page",
   "mail.reply": "Reply",
   "mail.replyAll": "Reply all",
   "mail.forward": "Forward",
@@ -93,6 +98,7 @@ export const en = {
   "compose.savingDraft": "Saving draft…",
   "compose.draftSaved": "Draft saved",
   "compose.noDrafts": "No drafts.",
+  "compose.otherClient": "other client",
   "compose.noRecipient": "(no recipient)",
   "compose.noSubject": "(no subject)",
   "compose.from": "From",
@@ -118,6 +124,7 @@ export const en = {
   "compose.fmtColor": "Text color",
   "compose.fmtHighlight": "Highlight color",
   "compose.fmtImage": "Insert image",
+  "compose.insertSignature": "Insert signature",
   "compose.linkPrompt": "Link URL:",
   "compose.imageTooLarge": "Image is too large (max 5 MB).",
   "compose.attachTooLarge": "File exceeds the 25 MB limit.",
@@ -172,6 +179,13 @@ export const en = {
   "settings.addAccount": "Add account",
   "settings.syncState": "State: {state}",
   "settings.confirmRemoveAccount": "Remove this account and all its mail?",
+  "settings.editAccount": "Edit account",
+  "settings.color": "Color",
+  "settings.serverSettings": "Change server settings / password",
+  "settings.serverSettingsHint": "Enter your mailbox password to apply server or password changes.",
+  "settings.signature": "Signature",
+  "settings.configureAccount": "Configure account",
+  "settings.editSignature": "Edit signature",
   "settings.theme": "Theme",
   "theme.light": "Light",
   "theme.dark": "Dark",