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/shared/state.js

@@ -25,6 +25,7 @@ const DEFAULT_STATE = {
     dustThresholdGwei: 100000,
     fraudContracts: [],
     tokenHolderCache: {},
+    theme: "system",
 };
 
 const state = {
@@ -55,6 +56,7 @@ async function saveState() {
         dustThresholdGwei: state.dustThresholdGwei,
         fraudContracts: state.fraudContracts,
         tokenHolderCache: state.tokenHolderCache,
+        theme: state.theme,
         currentView: state.currentView,
         selectedWallet: state.selectedWallet,
         selectedAddress: state.selectedAddress,
@@ -110,6 +112,7 @@ async function loadState() {
                 : 100000;
         state.fraudContracts = saved.fraudContracts || [];
         state.tokenHolderCache = saved.tokenHolderCache || {};
+        state.theme = saved.theme || "system";
         state.currentView = saved.currentView || null;
         state.selectedWallet =
             saved.selectedWallet !== undefined ? saved.selectedWallet : null;