revert: restore entire src/ from pre-doc-scanner commit (e4b44a7)

Going back to a known-good baseline. This version had:
- Working reveal mode with CSS Highlight API
- Working substitution (fetch + XHR hooks)
- Smart patterns (names, emails, phones, usernames)
- Encryption/sync (password, TOTP, WebAuthn)
- Multiple identity profiles
- Activity log
- Secret scanner
- Auto-detect PII warnings
- Pre-send PII detection

Kept current manifests (UUID, data_collection_permissions, version).
No renames applied — uses original naming (secretScanning, PPI, etc).
Will re-apply renames and new features from this working base.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

package.json

@@ -1,6 +1,6 @@
 {
   "name": "silent-send",
-  "version": "0.9.20",
+  "version": "0.9.21",
   "private": true,
   "license": "MIT",
   "description": "Browser extension that substitutes personal data before sending to AI services",