feat: custom domains for OpenWebUI + fix README issues

- Add custom domain support in Options page so users can add
  self-hosted AI services (e.g. https://ai.myserver.com)
- Background worker dynamically injects content scripts on
  custom domains using scripting.executeScript
- Add optional_host_permissions so Chrome can grant per-domain access
- Rewrite README: add clone step to Firefox instructions, clarify
  what "credentials" means in step 3, add Windows commands alongside
  Mac/Linux for every terminal step
- Bump version to 0.2.0

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/lib/storage.js

@@ -19,6 +19,7 @@ const DEFAULT_SETTINGS = {
   showHighlights: false,
   revealMode: false,
   maxLogEntries: 200,
+  customDomains: [],
   categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'general'],
 };