fix: Settings crash, photo preview before sending, incoming-content hardening

Settings crashed with a TypeError: the profile card passed a
SepotFingerprint object where the localized string formatter expects
a String (hidden behind a `dynamic` parameter). Also fixes RangeError
in avatar initials (empty names, double spaces) and switches initials
to grapheme clusters so emoji don't get split.

New photo preview screen: a captured or picked photo no longer sends
immediately — it opens in a preview with freehand drawing (7 colours,
undo), draggable text overlay, 90° rotation and a caption field.
Nothing leaves the device until Send is tapped.

Incoming-content hardening: decryption and content processing are now
separated in the delivery path. Once an envelope is decrypted the
ratchet has advanced, so redelivery can never help — permanently
malformed content (FormatException/TypeError) is acked and dropped
instead of poisoning the queue on every reconnect. Attachment metadata
now validates base64 of the crypto fields up front, and a corrupt
inline thumbnail no longer breaks bubble rendering.

PNG without real transparency (screenshots, edited photos) is now
re-encoded as JPEG — several times smaller on the wire.

Tests: 116 (new widget tests for Settings and the photo preview,
AttachmentMeta wire validation).

app/lib/providers/conversations_provider.dart

@@ -129,6 +129,7 @@ class ConversationsNotifier extends AsyncNotifier<List<Conversation>> {
     required Uint8List bytes,
     required String fileName,
     String? mime,
+    String? caption,
   }) async {
     final storage = ref.read(storageProvider);
     final service = ref.read(messagingServiceProvider);
@@ -140,6 +141,7 @@ class ConversationsNotifier extends AsyncNotifier<List<Conversation>> {
         bytes: bytes,
         fileName: fileName,
         mime: mime,
+        caption: caption,
       );
     } else {
       final contact = await _contact(contactId);
@@ -148,6 +150,7 @@ class ConversationsNotifier extends AsyncNotifier<List<Conversation>> {
         bytes: bytes,
         fileName: fileName,
         mime: mime,
+        caption: caption,
       );
     }