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/models/attachment.dart

@@ -97,6 +97,21 @@ class AttachmentMeta {
     if (size <= 0 || size > 512 * 1024 * 1024) {
       throw const FormatException('implausible attachment size');
     }
+    // Krypto pole MUSÍ být validní base64 (jinak je meta k ničemu);
+    // vyhodí FormatException → zpráva se zahodí jako vadná.
+    base64Decode(wire['k'] as String);
+    base64Decode(wire['n'] as String);
+    base64Decode(wire['d'] as String);
+    // Vadný náhled zprávu nezabíjí — UI ukáže placeholder a ostrá fotka
+    // se stáhne z blobu. base64Decode v build() by shodil celou bublinu.
+    var thumb = wire['th'] as String?;
+    if (thumb != null) {
+      try {
+        base64Decode(thumb);
+      } on FormatException {
+        thumb = null;
+      }
+    }
     return AttachmentMeta(
       blobId: wire['bid'] as String,
       keyB64: wire['k'] as String,
@@ -107,7 +122,7 @@ class AttachmentMeta {
       name: wire['name'] as String?,
       width: wire['w'] as int?,
       height: wire['h'] as int?,
-      thumbB64: wire['th'] as String?,
+      thumbB64: thumb,
       durationMs: wire['dur'] as int?,
       state: AttachmentTransferState.pending,
     );