fix: restore working content.js from v2.0.14 with renames applied

Reverted content.js to commit 9a11896 (the last version where reveal
mode worked) and re-applied only the renames:
- PPI → PII
- secretScanning → autoRedact
- SECRET_PATTERNS → REDACT_PATTERNS
- scanAndRedactSecrets → runAutoRedact
- category: 'secret' → category: 'redact'
- Added customRedactPatterns support to runAutoRedact

This restores:
- Individual word storage in sessionSubstitutions (needed for reveal)
- fetch(Request) handling (not just fetch(url, options))
- Blob/ArrayBuffer/URLSearchParams body conversion
- isInNonChatArea scoping (narrowed version)
- Proper noun detection gated by settings.detectProperNouns
- Mapping values added to configured skip set

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.17",
+  "version": "0.9.18",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "browser_specific_settings": {
     "gecko": {