feat: CSS Custom Highlight API — yellow for fake, terminal for real

Two visual modes using the Highlight API (zero DOM modification):

- Yellow highlight: fake/substituted values the AI received.
  Shows automatically in all AI responses, even without reveal mode.
  "The AI sees these yellow values, not your real data."

- Terminal style (black bg, green text): your real data shown
  in reveal mode. "This is YOUR data — the AI never saw this."

Uses CSS.highlights with ::highlight() pseudo-elements — no
spans, no DOM changes, no copy/paste artifacts. Falls back to
CSS class-based styling if Highlight API is unavailable.

Highlights are debounced (500ms) to handle streaming responses
without excessive reflows.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/content/content.css

@@ -13,13 +13,26 @@
   padding: 0 1px;
 }
 
-/* Revealed text styling (when reveal mode shows real values in responses) */
+/* CSS Custom Highlight API styles — zero DOM changes */
+
+/* Yellow highlight: fake values the AI received (non-reveal mode) */
+::highlight(ss-substituted) {
+  background-color: rgba(250, 204, 21, 0.4);
+  color: inherit;
+}
+
+/* Terminal style: real data shown in reveal mode (dark bg, green text) */
+::highlight(ss-revealed) {
+  background-color: rgba(0, 0, 0, 0.85);
+  color: #4ade80;
+}
+
+/* Fallback for browsers without Highlight API */
 .ss-revealed {
-  background: rgba(59, 130, 246, 0.1);
-  border-bottom: 2px solid rgba(59, 130, 246, 0.6);
-  border-radius: 2px;
+  background: rgba(0, 0, 0, 0.85);
+  color: #4ade80;
   padding: 0 2px;
-  cursor: help;
+  border-radius: 2px;
 }
 
 /* Floating reveal mode indicator */