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

src/options/options.html

@@ -629,7 +629,7 @@
     </section>
 
     <footer>
-      <p>Silent Send v0.9.17</p>
+      <p>Silent Send v0.9.18</p>
       <p style="font-size:11px;color:#9ca3af;margin-top:6px;max-width:600px">
         Silent Send is a convenience tool, not a security guarantee. Third-party sites may change how they send data at any time, which can cause missed substitutions without warning. You are responsible for verifying your data before sending. See the <a href="https://github.com/outis1one/silent-send/blob/main/LICENSE" target="_blank" style="color:#6b7280">LICENSE</a> for full terms.
       </p>