fix(extension): widen peerId generation to 16 hex chars (64-bit)

8 hex chars (32-bit) gives ~7% collision probability at the relay's
25k-peer capacity and ~1% at 10k peers. A same-room collision silently
triggers the dedup path and kicks the older session with a confusing
'Another session with this ID joined' error.

16 hex chars (64-bit) drops collision probability to ~1e-10 even at one
million peers. The server's JOIN_ROOM sanitizer already clamps peerId to
16 chars, so no server change is needed. Fully backward compatible:

- Existing clients keep their persisted 8-char peerId (only newly
  generated IDs change).
- Mixed 8-char + 16-char ID rooms work — both fit the server's 16-char
  clamp.
- No protocol change on the wire.

Birthday-paradox reference (computed, not estimated):
  32-bit @ 10k peers:  1.16%
  32-bit @ 25k peers:  7.02%
  64-bit @ 25k peers:  ~0.017e-9%

extension/background.js

@@ -339,7 +339,14 @@ function updateLocalPeerState(targetPeerId, updates) {
 async function getPeerId() {
     const data = await chrome.storage.local.get(['peerId']);
     if (data.peerId) return data.peerId;
-    const newId = self.crypto.randomUUID().substring(0, 8);
+    // 16 hex chars = 64 bits. At a busy relay (25k concurrent peers) the 32-bit
+    // (8-hex) generation would hit ~7% collision probability per snapshot —
+    // and a same-room collision triggers our dedup path, kicking the older
+    // session with a confusing error. 16 hex chars drops the probability to
+    // ~1e-10 even at a million peers, and the server already clamps peerId to
+    // 16 chars (server/index.js JOIN_ROOM sanitizer). Existing persisted 8-char
+    // IDs continue to work — this only affects newly-generated IDs.
+    const newId = self.crypto.randomUUID().replace(/-/g, '').substring(0, 16);
     await chrome.storage.local.set({ peerId: newId });
     return newId;
 }
@@ -1930,7 +1937,8 @@ async function handleAsyncMessage(message, sender, sendResponse) {
             sendResponse({ status: 'ok' });
         });
     } else if (message.type === 'REGENERATE_ID') {
-        const newId = self.crypto.randomUUID().substring(0, 8);
+        // Match getPeerId()'s 16-hex-char generation — see comment there.
+        const newId = self.crypto.randomUUID().replace(/-/g, '').substring(0, 16);
         chrome.storage.local.set({ peerId: newId }, () => {
             peerId = newId;
             addLog(`Identity regenerated: ${newId}`, 'success');