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/core/navigation.js

@@ -0,0 +1,31 @@
+// Depends on: core/utils.js ($)
+
+const PAGE_TITLES = {
+  dashboard: 'Overview',
+  connections: 'Virtual Hosts',
+  swarms: 'Server Tunnels',
+  'service-tunnels': 'Service Tunnels',
+  tabs: 'Proxy & CA',
+  ssh: 'SSH Connections',
+  rdp: 'Remote Desktop',
+  backups: 'Backups',
+  logs: 'Logs',
+  settings: 'Settings'
+};
+
+function navigateTo(page) {
+  document.querySelectorAll('.nav-item').forEach(i => i.classList.remove('active'));
+  document.querySelectorAll('.page').forEach(p => p.classList.remove('active'));
+  const navItem = document.querySelector(`.nav-item[data-page="${page}"]`);
+  if (navItem) navItem.classList.add('active');
+  const pageEl = $(`page-${page}`);
+  if (pageEl) pageEl.classList.add('active');
+  const titleEl = $('topbarTitle');
+  if (titleEl) titleEl.textContent = PAGE_TITLES[page] || page;
+}
+
+function setupNavigation() {
+  document.querySelectorAll('.nav-item').forEach(item => {
+    item.addEventListener('click', () => navigateTo(item.dataset.page));
+  });
+}