fix: restore missing isInNonChatArea + SKIP_REVEAL_TAGS from unmerged branch

Root cause found: commit 9a11896 (v2.0.14) added isInNonChatArea()
and expanded SKIP_REVEAL_TAGS with NAV/ASIDE/HEADER/FOOTER, but that
commit was on a branch that was never merged into main. When we started
our work from main, these fixes were missing.

Restored:
- isInNonChatArea() function — prevents reveal from touching sidebars,
  navigation, headers, footers, and other non-chat UI
- SKIP_REVEAL_TAGS expanded with NAV, ASIDE, HEADER, FOOTER
- isInNonChatArea checks added to revealInElement, unrevealInElement,
  and highlightMatches (both element-level and walker-level)

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "Silent Send",
-  "version": "0.9.14",
+  "version": "0.9.15",
   "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
   "permissions": [
     "storage",