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/widgets/avatar.dart

@@ -63,11 +63,17 @@ class SepotAvatar extends StatelessWidget {
   }
 
   String _getInitials(String name) {
-    final parts = name.trim().split(' ');
+    // Prázdné díly odfiltrovat — dvojitá mezera ve jméně nesmí shodit render.
+    // Grafémy (characters), ne code units — jméno začínající emoji by se
+    // jinak rozpůlilo na rozbitý surrogate.
+    final parts =
+        name.trim().split(' ').where((p) => p.isNotEmpty).toList();
     if (parts.length >= 2) {
-      return '${parts[0][0]}${parts[1][0]}'.toUpperCase();
+      return '${parts[0].characters.first}${parts[1].characters.first}'
+          .toUpperCase();
     }
-    return name.substring(0, name.length >= 2 ? 2 : 1).toUpperCase();
+    if (parts.isEmpty) return '?';
+    return parts[0].characters.take(2).toString().toUpperCase();
   }
 
   Color _colorFromName(String name) {