feat(w7): cosmetic room labels carried in the ticket (P5)

Rooms can now be named. A "Room name (optional)" field on the home Create card
mints a ticket carrying the label; PeerSpeakTicket gains a #[serde(default)]
`name` field (backward/forward compatible — serde ignores unknown fields and
defaults missing ones, so old/new builds still interoperate, just without
labels). restamp preserves the label so member-issued doors keep it; new
label_of helper reads it. Every member (creator or joiner) sets current_room.name
from the ticket, so presence reports a consistent "in <name>" to friends, and the
room-screen header shows the label under the wordmark. Labels are sanitized via
sanitize_name on both mint and display (untrusted peer-supplied ticket).

CoreCommand::Join gains room_name (used only when creating). +2 ticket tests
(label round-trip through restamp/label_of, pre-label backward-compat). clippy
--all-targets clean, 257 lib tests green.

Pure seam unit-tested + home field screenshot-verified; the in-room header label
and friend-side "in HangOut" presence display need a live/2-machine confirm.

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

src/core/messages.rs

@@ -6,7 +6,10 @@ use iroh::{EndpointAddr, EndpointId};
 
 #[derive(Debug, Clone)]
 pub enum CoreCommand {
-    Join { name: String, ticket: String, input_device: Option<String>, output_device: Option<String>, echo_cancellation: bool, avatar: crate::avatar::Avatar },
+    /// Join a room. `ticket` is "create" (or empty) to mint a fresh room, else a
+    /// share ticket to join. `room_name` is the creator's chosen cosmetic label
+    /// for a NEW room; it's ignored when joining (the label rides in the ticket).
+    Join { name: String, ticket: String, room_name: String, input_device: Option<String>, output_device: Option<String>, echo_cancellation: bool, avatar: crate::avatar::Avatar },
     Leave,
     ToggleMute,
     /// Change our avatar (W4) and re-announce it to the room over presence.