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/app_cs.arb

@@ -1698,5 +1698,41 @@
   "commonE2eBadgeFull": "End-to-end šifrováno",
   "@commonE2eBadgeFull": {
     "description": "Plná varianta badge šifrování (ikona zámku + text), zobrazuje se např. v hlavičce chatu / onboardingu."
+  },
+  "photoPreviewCaptionHint": "Přidat popisek…",
+  "@photoPreviewCaptionHint": {
+    "description": "hintText pole popisku na obrazovce náhledu fotky před odesláním"
+  },
+  "photoPreviewDraw": "Kreslit",
+  "@photoPreviewDraw": {
+    "description": "Tooltip nástroje kreslení prstem na náhledu fotky"
+  },
+  "photoPreviewAddText": "Přidat text",
+  "@photoPreviewAddText": {
+    "description": "Tooltip nástroje přidání textu na fotku"
+  },
+  "photoPreviewRotate": "Otočit",
+  "@photoPreviewRotate": {
+    "description": "Tooltip otočení fotky o 90°"
+  },
+  "photoPreviewUndo": "Zpět",
+  "@photoPreviewUndo": {
+    "description": "Tooltip vrácení posledního tahu kreslení"
+  },
+  "photoPreviewSend": "Poslat",
+  "@photoPreviewSend": {
+    "description": "Tooltip tlačítka odeslání fotky z náhledu"
+  },
+  "photoPreviewTextDialogTitle": "Text na fotce",
+  "@photoPreviewTextDialogTitle": {
+    "description": "Titulek dialogu pro zadání/úpravu textu na fotce"
+  },
+  "photoPreviewRemoveText": "Odebrat",
+  "@photoPreviewRemoveText": {
+    "description": "Tlačítko odebrání textu z fotky v dialogu úpravy"
+  },
+  "photoPreviewDecodeFailed": "Fotku se nepodařilo načíst",
+  "@photoPreviewDecodeFailed": {
+    "description": "SnackBar, když se vybraná fotka nedá dekódovat — náhled se zavře bez odeslání"
   }
-}
+}