feat: disable until configured + auto permission request for custom domains

Interception is now completely inactive until the user configures
at least one identity field or explicit mapping. Before that:
- Icon shows gray (unconfigured)
- No fetch/XHR hooks fire
- First-run banner tells user to set up

Custom domains: clicking "Add Domain" in Options now triggers
the browser's native permission prompt via permissions.request().
No more manual chrome://extensions site access step.

Icon states are now:
- Gray = unconfigured (nothing will happen)
- Black = active and protecting
- Blue = reveal mode on
- Red = manually disabled

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/options/options.html

@@ -121,7 +121,7 @@
         <button class="btn btn-primary" id="btnAddDomain">Add Domain</button>
       </div>
       <p class="section-desc" style="margin-top:8px;margin-bottom:0">
-        After adding a domain in Chrome, you also need to grant permission: <code>chrome://extensions/</code> → Silent Send → Details → Site access → add the domain.
+        When you click "Add Domain", your browser will ask you to confirm access. No extra steps needed.
       </p>
     </section>