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/core/navigation.js

@@ -1,4 +1,8 @@
-// Depends on: core/utils.js ($)
+/**
+ * Sidebar navigation for the dashboard.
+ * Manages the active nav item and visible page section, and wires up click listeners.
+ * Depends on: core/utils.js ($)
+ */
 
 const PAGE_TITLES = {
   dashboard: 'Overview',
@@ -13,6 +17,12 @@ const PAGE_TITLES = {
   settings: 'Settings'
 };
 
+/**
+ * Switch the dashboard to the specified page.
+ * Updates the active nav item, shows the corresponding `.page` section,
+ * and sets the topbar title.
+ * @param {string} page - Page key (e.g. `'dashboard'`, `'connections'`, `'ssh'`).
+ */
 function navigateTo(page) {
   document.querySelectorAll('.nav-item').forEach(i => i.classList.remove('active'));
   document.querySelectorAll('.page').forEach(p => p.classList.remove('active'));
@@ -24,6 +34,10 @@ function navigateTo(page) {
   if (titleEl) titleEl.textContent = PAGE_TITLES[page] || page;
 }
 
+/**
+ * Attach click listeners to all `.nav-item` elements.
+ * Called once during dashboard initialisation.
+ */
 function setupNavigation() {
   document.querySelectorAll('.nav-item').forEach(item => {
     item.addEventListener('click', () => navigateTo(item.dataset.page));