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

manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.20",
+  "version": "0.9.21",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "permissions": [
     "storage",