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.js

@@ -1,4 +1,5 @@
 import Storage from '../lib/storage.js';
+import api from '../lib/browser-polyfill.js';
 
 let mappings = [];
 let settings = {};
@@ -206,6 +207,20 @@ async function addDomain() {
     return;
   }
 
+  // Request browser permission for this domain
+  try {
+    const granted = await api.permissions.request({
+      origins: [domain + '/*'],
+    });
+    if (!granted) {
+      alert('Permission denied. The extension needs access to this domain to work.');
+      return;
+    }
+  } catch (e) {
+    // Firefox or older Chrome may not support optional permissions this way
+    console.warn('[Silent Send] Could not request permission:', e);
+  }
+
   domains.push(domain);
   settings.customDomains = domains;
   await Storage.saveSettings({ customDomains: domains });