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

src/options/options.html

@@ -629,7 +629,7 @@
     </section>
 
     <footer>
-      <p>Silent Send v0.9.14</p>
+      <p>Silent Send v0.9.15</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>