feat(app): redesigned interface, polls, drafts, forwarding and mentions

A new look across the app and a set of everyday messaging features.

Interface
- Home screen with Chats, Contacts and Settings tabs, a large title,
  search across chats and message text (diacritics-insensitive), filters
  and pinned chats.
- Conversation view with a translucent header and composer, grouped
  bubbles, author avatars and colours in groups, swipe to reply, an
  "N new messages" marker, a jump-to-latest button and in-chat search.
- Contact and group profiles: verification, shared photos, groups in
  common, disappearing messages, mute, block; members, admin badges,
  adding people, rename and leave for groups.
- Full-screen photo viewer that pages through the whole conversation.
- A stable avatar colour per person and a clearly visible read state.

Messaging
- Polls in chats and groups, single or multiple choice, with votes that
  can be changed or withdrawn; older clients see them as text.
- Drafts survive leaving a chat and show in the chat list.
- Forwarding of messages, photos, files and voice messages.
- Notes to self: a local-only, encrypted chat that never reaches the
  relay.
- @mentions in groups that notify even when the group is muted.
- Typing indicators sent as ephemeral messages, opt-in and mutual like
  read receipts.
- Delivery and read times per message and per group member.
- Profile photos, cropped, resized and stripped of metadata, delivered
  encrypted to contacts.
- Voice message playback at 1.5x and 2x.
- Call buttons in 1:1 chats and call log entries.

Fixes
- Muted groups no longer notify: muting was checked against the
  author's chat instead of the group.
- A blocked contact could unblock itself by changing its delivery
  address.
- Read receipts no longer regress to delivered when a late delivery
  receipt arrives.
- A message marked as failed corrects itself once it is delivered.
- Double ticks in groups now mean delivered to every member.

app/lib/core/l10n/error_messages.dart

@@ -15,7 +15,10 @@ String describeError(AppLocalizations l10n, Object error) => switch (error) {
       ContactLinkIncompleteError() => l10n.errorContactLinkIncomplete,
       AttachmentTooLargeError(:final maxMb) =>
         l10n.errorAttachmentTooLarge(maxMb),
+      AttachmentUnavailableError() => l10n.errorAttachmentUnavailable,
       GroupNeedsMemberError() => l10n.errorGroupNeedsMember,
+      NotGroupAdminError() => l10n.errorGroupNotAdmin,
+      LastGroupAdminError() => l10n.errorGroupLastAdmin,
       GroupTooLargeError(:final maxMembers) =>
         l10n.errorGroupMaxMembers(maxMembers),
       MemberNoDeliveryAddressError(:final displayName) =>