docs: add CONTRIBUTING.md, CHANGELOG.md, and JSDoc to entire codebase

Add docs/CONTRIBUTING.md covering the build system, dev workflow, all
npm scripts, how to add new native host message types, code style, and
debugging guidance.

Add CHANGELOG.md at the project root documenting all features and fixes
across the 1.0.0 release.

Add JSDoc (@param, @returns) to all previously undocumented exported
functions across 35 JS files:
- native-host/holesail-manager/ (index, virtual-hosts, service-tunnels,
  servers, port-allocator)
- native-host top-level managers (startup, connect-proxy, https-proxy,
  certificate-authority, ssh-manager, rdp-manager)
- extension/background/ (logs, native-messaging, proxy, message-router)
- extension/dashboard/core/ (utils, navigation, init)
- extension/dashboard/ui/ (modal, toast, state-tag)
- extension/dashboard/pages/ (all 10 page files)
- extension/dashboard/refresh.js, events.js
- extension/dashboard/data/hostname-validator.js
- scripts/ (build-host, run-install)

extension/dashboard/pages/settings.js

@@ -1,5 +1,13 @@
-// Depends on: core/utils.js ($), core/state.js (settings, SETTINGS_DEFAULTS), ui/toast.js (showToast)
+/**
+ * Settings page — syncs the settings form with the persisted settings object
+ * and saves changes to the native host.
+ * Depends on: core/utils.js ($), core/state.js (settings, SETTINGS_DEFAULTS), ui/toast.js (showToast)
+ */
 
+/**
+ * Sync all settings form controls with the current `settings` object.
+ * Called on every refresh cycle to keep the UI in sync with persisted state.
+ */
 function updateSettingsUI() {
   $('toggleNotify')?.classList.toggle('active', settings.notifyOnDisconnect === true);
   $('toggleNotifyTunnelError')?.classList.toggle('active', settings.notifyOnTunnelError !== false);