Support nebula:// internal pages & GPU tools

Introduce support for an internal nebula:// URL scheme and internal page routing (ResolveInternalUrl / ToInternalUrl), including dedicated slugs for home, settings, downloads, big-picture, gpu-diagnostics, insecure and a 404 fallback. Wire internal resolution into browser creation and tab navigation so internal pages load from local UI files. Add an insecure-warning interstitial flow with a navigate-insecure command and a one-shot bypass set (ShouldBypassInsecureWarning) so content can request navigating to an HTTP target after user confirmation. Harden BrowserClient handling to resolve Chromium new-tab and nebula internal URLs, redirect HTTP to the insecure warning when appropriate, and handle 404 responses by loading the internal 404 page. Update chrome UI behavior to hide internal home URLs, accept nebula:// in navigation input checks, and add a GPU Diagnostics page (revamped UI + diagnostic scripts) plus menu entry. Misc: improve URL utilities (scheme checks, percent-encoding, decorations), fix 404 display text, adjust menu popup size, tweak window frame styling (DWM attributes) and remove branding block from chrome UI CSS.

ui/pages/insecure.html

@@ -62,6 +62,10 @@
   const box = document.getElementById('targetBox');
   if (target) box.textContent = target;
   function sendNavigate(url, opts){
+    if (opts && opts.insecureBypass && window.nebulaNative && window.nebulaNative.postMessage){
+      window.nebulaNative.postMessage('navigate-insecure', url);
+      return;
+    }
     if (window.electronAPI && window.electronAPI.sendToHost){
       window.electronAPI.sendToHost('navigate', url, opts||{});
     } else if (window.parent && window.parent !== window) {