fix: disable proper noun detection by default (too many false positives)

Proper noun detection (flagging capitalized phrases like "Getting
Started", "Generate Design", "Introduction Getting Started") now
disabled by default. Enable via popup → Options tab → Detect proper
nouns.

The pattern-based detection (IPs, emails, API keys, addresses, paths,
etc.) remains always-on and reliable. The capitalized phrase heuristic
was producing too many false positives on normal UI phrases, headings,
and instructions — adding words to the filter was a losing game.

Added detectProperNouns toggle to popup Options tab.
Updated README to note it's opt-in.

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/popup/popup.html

@@ -236,6 +236,14 @@
         <label class="toggle"><input type="checkbox" id="optDocPreview" checked><span class="toggle-slider"></span></label>
       </div>
 
+      <div class="setting-item">
+        <div class="setting-label">
+          <strong>Detect proper nouns</strong>
+          <span class="setting-desc">Flag capitalized phrases (names, companies) — may produce false positives</span>
+        </div>
+        <label class="toggle"><input type="checkbox" id="optProperNouns"><span class="toggle-slider"></span></label>
+      </div>
+
       <div style="margin-top:12px;padding-top:10px;border-top:1px solid #e5e7eb">
         <button class="btn" id="btnOpenFullOptions" style="width:100%;font-size:12px">Open Full Options Page</button>
         <p class="help-text" style="margin-top:6px;text-align:center">Sync, encryption, org, version history, import/export, and more</p>

src/popup/popup.js

@@ -225,6 +225,7 @@ async function initUnlockedUI() {
   $('#optAutoRedact').checked = settings.autoRedactDetected !== false;
   $('#optHighlights').checked = settings.showHighlights || false;
   $('#optDocPreview').checked = settings.docScanPreview !== false;
+  $('#optProperNouns').checked = settings.detectProperNouns || false;
 
   // Options tab change handlers
   const optHandlers = [
@@ -233,6 +234,7 @@ async function initUnlockedUI() {
     ['optAutoRedact', 'autoRedactDetected'],
     ['optHighlights', 'showHighlights'],
     ['optDocPreview', 'docScanPreview'],
+    ['optProperNouns', 'detectProperNouns'],
   ];
   for (const [id, key] of optHandlers) {
     $(`#${id}`).addEventListener('change', async (e) => {