fix(ui): disable modal backdrop blur under software rendering (#1092)

With hardware acceleration disabled, Chrome rasterizes in software and a
full-viewport backdrop-filter forces a per-frame CPU blur over everything
behind the modal, freezing the whole browser.

Detect software rendering via the unmasked WebGL renderer string at app
startup and drop the backdrop blur in that case. Also route the download
modal's sticky toolbar through the shared --modal-backdrop-blur variable
instead of a hardcoded blur(8px).

static/js/core.js

@@ -12,6 +12,7 @@ import { helpManager } from './managers/HelpManager.js';
 import { doctorManager } from './managers/DoctorManager.js';
 import { bannerService } from './managers/BannerService.js';
 import { initTheme, initBackToTop } from './utils/uiHelpers.js';
+import { applyModalBackdropBlurPolicy } from './utils/renderingCapability.js';
 import { initializeInfiniteScroll } from './utils/infiniteScroll.js';
 import { i18n } from './i18n/index.js';
 import { onboardingManager } from './managers/OnboardingManager.js';
@@ -33,7 +34,11 @@ export class AppCore {
         if (this.initialized) return;
 
         console.log('AppCore: Initializing...');
-        
+
+        // Disable full-viewport backdrop blur under software rendering before
+        // anything can open a modal (issue #1092)
+        applyModalBackdropBlurPolicy();
+
         // Initialize i18n first
         window.i18n = i18n;
         // Wait for i18n to be ready