feat: add reveal paste-back tool + fix smart detection bail

Test tab now has two modes:
- Strip (real → fake): paste text with real data, see what gets sent
- Reveal (fake → real): paste AI output with fake data, get back
  real data with a "Copy to Clipboard" button

Also fixes:
- content.js smartSubstitute bailing when identity.enabled was
  undefined (defaulted enabled to all-true instead of returning)
- Test tab now reloads identity from storage on tab switch so
  changes saved in the Identity tab take effect immediately
- Shows yellow warning when identity fields are missing

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/popup/popup.css

@@ -408,6 +408,48 @@ body {
   color: #059669;
 }
 
+/* Test tab mode toggle */
+.test-mode-toggle {
+  display: flex;
+  background: #f3f4f6;
+  border-radius: 6px;
+  padding: 2px;
+  margin-bottom: 10px;
+}
+
+.test-mode-btn {
+  flex: 1;
+  padding: 6px 0;
+  border: none;
+  border-radius: 4px;
+  font-size: 11px;
+  font-weight: 500;
+  cursor: pointer;
+  background: transparent;
+  color: #6b7280;
+  transition: all 0.15s;
+}
+
+.test-mode-btn.active {
+  background: #fff;
+  color: #111;
+  box-shadow: 0 1px 2px rgba(0,0,0,0.1);
+}
+
+.test-identity-status {
+  margin-top: 8px;
+  padding: 6px 10px;
+  background: #fef3c7;
+  border-radius: 6px;
+  font-size: 11px;
+  color: #92400e;
+  display: none;
+}
+
+.test-identity-status.visible {
+  display: block;
+}
+
 /* Test tab */
 .help-text {
   font-size: 12px;