Fix Discord token auto-auth fallback

[ZARAEM]

Summary

• Move Discord token extraction into a focused helper used by the content script.
• Retry auth token lookup after dispatching beforeunload, which matches Discord's token flush behavior used by the bundled Classic extension.
• Add a same-origin iframe storage fallback for cases where the page's direct localStorage surface is unavailable.
• Add regression coverage for direct storage tokens, beforeunload-flushed tokens, and iframe storage fallback.

Root cause

Extension auto-auth only checked direct page localStorage, so Discord changes that keep or expose the token differently could leave Discrub unable to retrieve a token even while the user is logged into discord.com.

Fixes #9.

Validation

• npm test -- src/extension/content.test.ts src/extension/tokenExtraction.test.ts
• npm test -- src/extension/background.test.ts src/extension/messaging.test.ts src/extension/integration.test.ts
• npm run build:extension:chrome
• npm run build:extension:firefox
• npm run build