Add host platform detection and macOS styles

Expose host platform to the UI and apply platform-specific styling. Backend: add CurrentPlatformName() and include "platform" in the JSON sent from NebulaController so the frontend can know the host OS. Frontend: detectHostPlatform() initializes state.platform, applyPlatform() sets a platform-* body class, and applyState() applies it. CSS: add .platform-macos rules to adjust title padding and hide window controls on macOS. Also fix FilePathToUrl to avoid producing an extra slash when the encoded path already starts with '/'.

ui/css/chrome.css

@@ -89,6 +89,10 @@ button:disabled {
   background: var(--bg);
 }
 
+.platform-macos .title-row {
+  padding: 0 12px 0 78px;
+}
+
 /* ── Tabs ───────────────────────────────────────────────────── */
 
 .tabs {
@@ -257,6 +261,10 @@ button:disabled {
   color: white;
 }
 
+.platform-macos .window-controls {
+  display: none;
+}
+
 /* ── Toolbar ────────────────────────────────────────────────── */
 
 .toolbar {

ui/js/chrome.js

@@ -28,9 +28,24 @@ const state = {
   canGoBack: false,
   canGoForward: false,
   favicon: '',
+  platform: detectHostPlatform(),
   tabs: []
 };
 
+function detectHostPlatform() {
+  const platform = `${navigator.userAgentData?.platform || ''} ${navigator.platform || ''} ${navigator.userAgent || ''}`.toLowerCase();
+  if (platform.includes('mac')) return 'macos';
+  if (platform.includes('win')) return 'windows';
+  if (platform.includes('linux')) return 'linux';
+  return 'unknown';
+}
+
+function applyPlatform(platform) {
+  const normalized = ['macos', 'windows', 'linux'].includes(platform) ? platform : 'unknown';
+  document.body.classList.remove('platform-macos', 'platform-windows', 'platform-linux', 'platform-unknown');
+  document.body.classList.add(`platform-${normalized}`);
+}
+
 function hexToRgb(hex) {
   if (!hex || typeof hex !== 'string') return null;
   let normalized = hex.trim().replace(/^#/, '');
@@ -191,6 +206,7 @@ function renderTabs() {
 
 function applyState(nextState) {
   Object.assign(state, nextState || {});
+  applyPlatform(state.platform);
 
   const title = state.title || 'New Tab';
   const url = state.url || '';