feat: screen sharing via pixelpass (Discord-style, presence-borne ticket)

Surface pixelpass screen-sharing from inside a peerspeak room. peerspeak owns
voice, pixelpass owns pixels — they're never Cargo deps of each other; the
contract is pixelpass's CLI flags + its `--output json` stdout stream.

Modelled on Discord: multiple simultaneous sharers, a 🔴 Live badge + 👁 Watch
on each sharing peer's card, and in-progress shares visible to late joiners.

- New `src/screenshare` module: pure `parse_pixelpass_event` seam + `pixelpass_path`
  discovery (13 unit tests), async `spawn_host` (→ ticket) and `spawn_viewer`
  (→ parse connected{url} → open mpv, vlc fallback). No new deps.
- Sharing rides presence: `PeerState.sharing: Option<ticket>` (serde-defaulted),
  so the existing gossip re-announce delivers the offer to late joiners for free
  and a PeerUpdated fires on start/stop — no separate gossip message needed.
- core: Start/Stop/ViewShare commands; host + viewer children tracked in the
  session, killed on stop/leave (kill_on_drop backstop). Viewer limit left to
  pixelpass's bandwidth-measured cap.
- UI: Share/Stop button (graceful "needs pixelpass" disabled state), Live badge
  + Watch on peer cards, Sharing badge on the self card. Verified by screenshot.
- config: optional `pixelpass_path` override (hand-editable).

Tests-green; the 2-machine gossip/remote path is not yet field-verified.

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

src/bin/test_net.rs

@@ -61,6 +61,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         name: "Alice".to_string(),
         is_muted: false,
         addr: endpoint_a.addr(),
+        sharing: None,
     };
     room_a.join(&ticket_str, state_a).await?;
     println!("Node A joined topic.");
@@ -78,6 +79,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         name: "Bob".to_string(),
         is_muted: false,
         addr: endpoint_b.addr(),
+        sharing: None,
     };
     room_b.join(&ticket_str, state_b).await?;
     println!("Node B joined topic.");