feat: add theme setting (Light/Dark/System) with dark mode

Add theme preference (light/dark/system) stored in extension state.
System mode follows prefers-color-scheme and listens for changes.
Dark mode inverts the monochrome palette (white-on-black).
Theme selector added to Display section in settings.

Closes #125

src/popup/styles/main.css

@@ -15,6 +15,18 @@
     --color-section: #dddddd;
 }
 
+html.dark {
+    --color-bg: #000000;
+    --color-fg: #ffffff;
+    --color-muted: #999999;
+    --color-border: #ffffff;
+    --color-border-light: #333333;
+    --color-hover: #111111;
+    --color-well: #0a0a0a;
+    --color-danger-well: #1a0000;
+    --color-section: #222222;
+}
+
 body {
     width: 396px;
     overflow-x: hidden;