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/ui/toast.js

@@ -1,7 +1,15 @@
-// Depends on: core/utils.js ($)
+/**
+ * Toast notification and clipboard copy helpers for the dashboard.
+ * Depends on: core/utils.js ($)
+ */
 
 let toastTimer = null;
 
+/**
+ * Show a toast notification for 2.8 seconds.
+ * @param {string} msg - Message to display.
+ * @param {'default'|'success'|'error'} [type='default'] - Visual style class.
+ */
 function showToast(msg, type = 'default') {
   const el = $('toast');
   if (!el) return;
@@ -11,6 +19,12 @@ function showToast(msg, type = 'default') {
   toastTimer = setTimeout(() => { el.className = 'toast'; }, 2800);
 }
 
+/**
+ * Copy text to the clipboard and show a success toast.
+ * Briefly adds a `copied` class to `btnEl` for visual feedback.
+ * @param {string} text - Text to copy.
+ * @param {HTMLElement} [btnEl] - Optional button element to animate.
+ */
 function copyToClipboard(text, btnEl) {
   navigator.clipboard.writeText(text).then(() => {
     if (btnEl) {