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.dart

@@ -2197,6 +2197,60 @@ abstract class AppLocalizations {
   /// In cs, this message translates to:
   /// **'End-to-end šifrováno'**
   String get commonE2eBadgeFull;
+
+  /// hintText pole popisku na obrazovce náhledu fotky před odesláním
+  ///
+  /// In cs, this message translates to:
+  /// **'Přidat popisek…'**
+  String get photoPreviewCaptionHint;
+
+  /// Tooltip nástroje kreslení prstem na náhledu fotky
+  ///
+  /// In cs, this message translates to:
+  /// **'Kreslit'**
+  String get photoPreviewDraw;
+
+  /// Tooltip nástroje přidání textu na fotku
+  ///
+  /// In cs, this message translates to:
+  /// **'Přidat text'**
+  String get photoPreviewAddText;
+
+  /// Tooltip otočení fotky o 90°
+  ///
+  /// In cs, this message translates to:
+  /// **'Otočit'**
+  String get photoPreviewRotate;
+
+  /// Tooltip vrácení posledního tahu kreslení
+  ///
+  /// In cs, this message translates to:
+  /// **'Zpět'**
+  String get photoPreviewUndo;
+
+  /// Tooltip tlačítka odeslání fotky z náhledu
+  ///
+  /// In cs, this message translates to:
+  /// **'Poslat'**
+  String get photoPreviewSend;
+
+  /// Titulek dialogu pro zadání/úpravu textu na fotce
+  ///
+  /// In cs, this message translates to:
+  /// **'Text na fotce'**
+  String get photoPreviewTextDialogTitle;
+
+  /// Tlačítko odebrání textu z fotky v dialogu úpravy
+  ///
+  /// In cs, this message translates to:
+  /// **'Odebrat'**
+  String get photoPreviewRemoveText;
+
+  /// SnackBar, když se vybraná fotka nedá dekódovat — náhled se zavře bez odeslání
+  ///
+  /// In cs, this message translates to:
+  /// **'Fotku se nepodařilo načíst'**
+  String get photoPreviewDecodeFailed;
 }
 
 class _AppLocalizationsDelegate

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';
 }

app/lib/l10n/generated/app_localizations_de.dart

@@ -1270,4 +1270,31 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get commonE2eBadgeFull => 'Ende-zu-Ende-verschlüsselt';
+
+  @override
+  String get photoPreviewCaptionHint => 'Bildunterschrift hinzufügen…';
+
+  @override
+  String get photoPreviewDraw => 'Zeichnen';
+
+  @override
+  String get photoPreviewAddText => 'Text hinzufügen';
+
+  @override
+  String get photoPreviewRotate => 'Drehen';
+
+  @override
+  String get photoPreviewUndo => 'Rückgängig';
+
+  @override
+  String get photoPreviewSend => 'Senden';
+
+  @override
+  String get photoPreviewTextDialogTitle => 'Text auf dem Foto';
+
+  @override
+  String get photoPreviewRemoveText => 'Entfernen';
+
+  @override
+  String get photoPreviewDecodeFailed => 'Foto konnte nicht geladen werden';
 }

app/lib/l10n/generated/app_localizations_en.dart

@@ -1243,4 +1243,31 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get commonE2eBadgeFull => 'End-to-end encrypted';
+
+  @override
+  String get photoPreviewCaptionHint => 'Add a caption…';
+
+  @override
+  String get photoPreviewDraw => 'Draw';
+
+  @override
+  String get photoPreviewAddText => 'Add text';
+
+  @override
+  String get photoPreviewRotate => 'Rotate';
+
+  @override
+  String get photoPreviewUndo => 'Undo';
+
+  @override
+  String get photoPreviewSend => 'Send';
+
+  @override
+  String get photoPreviewTextDialogTitle => 'Text on photo';
+
+  @override
+  String get photoPreviewRemoveText => 'Remove';
+
+  @override
+  String get photoPreviewDecodeFailed => 'Couldn\'t load the photo';
 }

app/lib/l10n/generated/app_localizations_it.dart

@@ -1257,4 +1257,31 @@ class AppLocalizationsIt extends AppLocalizations {
 
   @override
   String get commonE2eBadgeFull => 'Cifrato end-to-end';
+
+  @override
+  String get photoPreviewCaptionHint => 'Aggiungi una didascalia…';
+
+  @override
+  String get photoPreviewDraw => 'Disegna';
+
+  @override
+  String get photoPreviewAddText => 'Aggiungi testo';
+
+  @override
+  String get photoPreviewRotate => 'Ruota';
+
+  @override
+  String get photoPreviewUndo => 'Annulla';
+
+  @override
+  String get photoPreviewSend => 'Invia';
+
+  @override
+  String get photoPreviewTextDialogTitle => 'Testo sulla foto';
+
+  @override
+  String get photoPreviewRemoveText => 'Rimuovi';
+
+  @override
+  String get photoPreviewDecodeFailed => 'Impossibile caricare la foto';
 }

app/lib/l10n/generated/app_localizations_ru.dart

@@ -1282,4 +1282,31 @@ class AppLocalizationsRu extends AppLocalizations {
 
   @override
   String get commonE2eBadgeFull => 'Сквозное шифрование';
+
+  @override
+  String get photoPreviewCaptionHint => 'Добавить подпись…';
+
+  @override
+  String get photoPreviewDraw => 'Рисовать';
+
+  @override
+  String get photoPreviewAddText => 'Добавить текст';
+
+  @override
+  String get photoPreviewRotate => 'Повернуть';
+
+  @override
+  String get photoPreviewUndo => 'Отменить';
+
+  @override
+  String get photoPreviewSend => 'Отправить';
+
+  @override
+  String get photoPreviewTextDialogTitle => 'Текст на фото';
+
+  @override
+  String get photoPreviewRemoveText => 'Удалить';
+
+  @override
+  String get photoPreviewDecodeFailed => 'Не удалось загрузить фото';
 }