feat(w7): recently-joined rooms list with one-click rejoin (P5)

Add a purely-local, most-recent-first recents list so users can hop back
into a room they were just in — meaningful now that rooms carry cosmetic
labels.

- src/recents.rs (new): `Recent {name, ticket, joined_at}`, `push_recent`
  (de-dupes by room `topic_id`, refresh-and-move-to-front, caps at
  RECENTS_MAX=12), `remove_recent`, `relative_time` ("5m ago"). 6 tests.
- PeerSpeakTicket::topic_of — the stable room identity used as the de-dup
  key (host addr + label change between members/sessions; topic doesn't).
- AppConfig.recents (`#[serde(default)]`, back-compat) — local UI state,
  never sent over the wire.
- Recorded on RoomJoined (label via label_of); rendered as a "Recent
  rooms" block in connect_card (each entry → JoinRecent, ✕ → RemoveRecent),
  shown only when non-empty.

Rejoin is best-effort by design: the stored ticket only admits us while
the room is still live and reachable (reliability is P6 discovery + the
member-issued ticket floor, not this list).

263 lib tests green, clippy --all-targets clean. Recents UI
screenshot-verified (seeded config → ages + Untitled-room fallback render).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/network/mod.rs

@@ -114,6 +114,14 @@ impl PeerSpeakTicket {
     pub fn label_of(ticket_str: &str) -> String {
         ticket_str.parse::<PeerSpeakTicket>().map(|t| t.name).unwrap_or_default()
     }
+
+    /// The room's `topic_id` embedded in a ticket string, or `None` if the ticket
+    /// can't be parsed. Pure; used as the stable room identity for de-duplicating
+    /// the recents list (the host address and label change between members/sessions,
+    /// but the topic uniquely identifies the gathering).
+    pub fn topic_of(ticket_str: &str) -> Option<[u8; 32]> {
+        ticket_str.parse::<PeerSpeakTicket>().ok().map(|t| t.topic_id)
+    }
 }
 
 impl std::fmt::Display for PeerSpeakTicket {
@@ -251,6 +259,10 @@ mod tests {
         assert_eq!(PeerSpeakTicket::label_of(&restamped), "HangOut");
         // An unparseable ticket has no label rather than panicking.
         assert_eq!(PeerSpeakTicket::label_of("not-a-ticket"), "");
+        // topic_of reads the room identity, and returns None for a bad ticket.
+        assert_eq!(PeerSpeakTicket::topic_of(&labelled), Some(topic_id));
+        assert_eq!(PeerSpeakTicket::topic_of(&restamped), Some(topic_id));
+        assert_eq!(PeerSpeakTicket::topic_of("not-a-ticket"), None);
         // Backward-compat: a pre-label ticket JSON (no `name` key) still parses,
         // defaulting the label to "".
         let legacy_json = serde_json::json!({