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/l10n/generated/app_localizations_cs.dart

@@ -1248,4 +1248,31 @@ class AppLocalizationsCs extends AppLocalizations {
 
   @override
   String get commonE2eBadgeFull => 'End-to-end šifrováno';
+
+  @override
+  String get photoPreviewCaptionHint => 'Přidat popisek…';
+
+  @override
+  String get photoPreviewDraw => 'Kreslit';
+
+  @override
+  String get photoPreviewAddText => 'Přidat text';
+
+  @override
+  String get photoPreviewRotate => 'Otočit';
+
+  @override
+  String get photoPreviewUndo => 'Zpět';
+
+  @override
+  String get photoPreviewSend => 'Poslat';
+
+  @override
+  String get photoPreviewTextDialogTitle => 'Text na fotce';
+
+  @override
+  String get photoPreviewRemoveText => 'Odebrat';
+
+  @override
+  String get photoPreviewDecodeFailed => 'Fotku se nepodařilo načíst';
 }