efactor(extension): modularize dashboard, background, and docs

Split monolithic dashboard.js (2753 lines) into 18 focused modules
under dashboard/{core,data,ui,pages}/ with refresh.js and events.js
as orchestrators. Extracted ~900-line inline <style> into dashboard.css
and moved dashboard.html to dashboard/dashboard.html.

Split background.js (609 lines) into background/{logs,state,proxy,
native-messaging,tab-lifecycle,message-router}.js with a thin entry
point using importScripts().

Deleted dead files: wrong-domain.js (duplicate of inline script).
Updated manifest.json web_accessible_resources for new paths.
Updated docs/ARCHITECTURE.md to reflect the new file structure.

No functionality changed. No build step introduced.

extension/dashboard/ui/toast.js

@@ -0,0 +1,22 @@
+// Depends on: core/utils.js ($)
+
+let toastTimer = null;
+
+function showToast(msg, type = 'default') {
+  const el = $('toast');
+  if (!el) return;
+  el.textContent = msg;
+  el.className = 'toast show' + (type !== 'default' ? ' ' + type : '');
+  clearTimeout(toastTimer);
+  toastTimer = setTimeout(() => { el.className = 'toast'; }, 2800);
+}
+
+function copyToClipboard(text, btnEl) {
+  navigator.clipboard.writeText(text).then(() => {
+    if (btnEl) {
+      btnEl.classList.add('copied');
+      setTimeout(() => btnEl.classList.remove('copied'), 1500);
+    }
+    showToast('Copied to clipboard', 'success');
+  }).catch(() => showToast('Copy failed', 'error'));
+}