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/index.js

@@ -6,6 +6,7 @@ const { state, saveState, loadState } = require("../shared/state");
 const { refreshPrices } = require("../shared/prices");
 const { refreshBalances } = require("../shared/balances");
 const { $, showView } = require("./views/helpers");
+const { applyTheme } = require("./theme");
 
 const home = require("./views/home");
 const welcome = require("./views/welcome");
@@ -176,6 +177,7 @@ async function init() {
     }
 
     await loadState();
+    applyTheme(state.theme);
 
     // Auto-default active address
     if (