feat: implement photo albums (media groups) and persist account selection

Group photos with shared media_album_id into single album bubbles with
grid layout (up to 3x cols). Album navigation treats grouped photos as
one unit (j/k skip entire album). Persist selected account to
accounts.toml so it survives app restart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/helpers/test_data.rs

@@ -115,6 +115,7 @@ pub struct TestMessageBuilder {
     reply_to: Option<ReplyInfo>,
     forward_from: Option<ForwardInfo>,
     reactions: Vec<ReactionInfo>,
+    media_album_id: i64,
 }
 
 impl TestMessageBuilder {
@@ -134,6 +135,7 @@ impl TestMessageBuilder {
             reply_to: None,
             forward_from: None,
             reactions: vec![],
+            media_album_id: 0,
         }
     }
 
@@ -187,8 +189,13 @@ impl TestMessageBuilder {
         self
     }
 
+    pub fn media_album_id(mut self, id: i64) -> Self {
+        self.media_album_id = id;
+        self
+    }
+
     pub fn build(self) -> MessageInfo {
-        MessageInfo::new(
+        let mut msg = MessageInfo::new(
             MessageId::new(self.id),
             self.sender_name,
             self.is_outgoing,
@@ -203,7 +210,9 @@ impl TestMessageBuilder {
             self.reply_to,
             self.forward_from,
             self.reactions,
-        )
+        );
+        msg.metadata.media_album_id = self.media_album_id;
+        msg
     }
 }