feat: identicons, server fingerprints, lock status (TOFU)

Identicon generator:
- Deterministic 5x5 symmetric pattern from fingerprint hash
- HSL-derived colors, rendered as inline SVG
- Click any identicon to copy its fingerprint to clipboard
- Used for participants, user identity, and relay servers

Server identity (TOFU — Trust On First Use):
- Ping returns server fingerprint (QUIC peer certificate hash)
- First contact: auto-saved as known fingerprint
- Subsequent pings: compared against known fingerprint
- Lock icons: locked (verified), unlocked (new), warning (changed), red (offline)
- Fingerprint mismatch shows confirmation dialog before connecting

UI updates:
- Participants show identicons instead of letter avatars
- User identity shows identicon + fingerprint on connect screen
- Manage Relays shows identicon per server with lock status
- Relay button shows lock icon instead of colored dot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/index.html

@@ -37,6 +37,7 @@
           <p id="connect-error" class="error"></p>
         </div>
         <div class="identity-info">
+          <span id="my-identicon"></span>
           <span id="my-fingerprint" class="fp-display"></span>
         </div>
         <div class="recent-rooms" id="recent-rooms"></div>