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/content/content.js

@@ -604,9 +604,11 @@
     }
 
     // Proper noun heuristic — catch names, company names, project names
-    // that aren't configured in identity
-    const properNouns = detectProperNouns(text, configured);
-    findings.push(...properNouns);
+    // Disabled by default (too many false positives). Enable in Options.
+    if (settings.detectProperNouns) {
+      const properNouns = detectProperNouns(text, configured);
+      findings.push(...properNouns);
+    }
 
     // Deduplicate by value
     const seen = new Set();