fix: unreveal losing case — 'Ademo Demo' became 'ademo demo'

sessionSubstitutions stored keys as toLowerCase() for lookup, but the
lowercase key was also used as the substitute value when building
reveal pairs. This caused unrevealText to replace "John Smith" with
"ademo demo" instead of "Ademo Demo".

Fixed by storing both the original-case substitute and the original
real value in sessionSubstitutions. The lowercase key is still used
for lookup, but the preserved-case value is used for reveal pairs.

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/content/content.js

@@ -661,7 +661,11 @@
     // Record what was actually substituted so reveal knows
     for (const r of replacements) {
       if (r.replaced && r.original) {
-        sessionSubstitutions.set(r.replaced.toLowerCase(), r.original);
+        // Store with lowercase key for lookup, but preserve original case
+        sessionSubstitutions.set(r.replaced.toLowerCase(), {
+          original: r.original,
+          replaced: r.replaced,  // preserve original case
+        });
       }
     }
 
@@ -1272,7 +1276,8 @@
     // Helper: only add if this substitute was actually sent
     function addIfUsed(from, to, caseSensitive) {
       if (!from || !to) return;
-      if (sessionSubstitutions.has(from.toLowerCase())) {
+      const entry = sessionSubstitutions.get(from.toLowerCase());
+      if (entry) {
         pairs.push({ from, to, caseSensitive });
       }
     }
@@ -1301,9 +1306,9 @@
     }
 
     // Also add auto-detect and secret scanner substitutions from this session
-    for (const [replaced, original] of sessionSubstitutions) {
-      if (!pairs.some(p => p.from.toLowerCase() === replaced)) {
-        pairs.push({ from: replaced, to: original });
+    for (const [key, entry] of sessionSubstitutions) {
+      if (!pairs.some(p => p.from.toLowerCase() === key)) {
+        pairs.push({ from: entry.replaced, to: entry.original });
       }
     }