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 {