feat(ui): direct-only mode setting (no relay fallback)

New toggle in Settings → "Direct-only mode (no relay fallback)":
- Default: OFF (normal behavior, relay fallback on P2P failure)
- When ON: connect returns error if P2P fails, with full
  candidate_diags in the debug log showing why each candidate
  failed. Call never falls back to relay.

Useful for testing NAT traversal — you see the exact failure
reason instead of the call silently working through relay.

Wired end-to-end:
- Settings.directOnly persisted in localStorage
- Passed as directOnly param to Rust connect command
- connect:path_negotiated shows direct_only flag
- connect:direct_only_failed emits on failure with diags

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

desktop/index.html

@@ -187,6 +187,10 @@
               <input id="s-call-debug" type="checkbox" />
               Call flow debug logs (trace every step of a call)
             </label>
+            <label class="checkbox">
+              <input id="s-direct-only" type="checkbox" />
+              Direct-only mode (no relay fallback — fails if P2P can't connect)
+            </label>
           </div>
           <div class="settings-section" id="s-call-debug-section" style="display:none">
             <h3>Call Debug Log</h3>