perf(ui): eliminate per-frame ImageFilter.blur sigma animation in showAppDialog backdrop #1341

Problem

In lib/shared/widgets/app_dialog.dart, _DialogBackdrop renders the modal frosted backdrop with animated Gaussian blur sigma on every frame of the enter/exit transition (200 ms):

BackdropFilter(
  filter: ImageFilter.blur(sigmaX: sigma * t, sigmaY: sigma * t),
  child: ColoredBox(
    color: const Color(0xFF000000).withValues(alpha: tintAlpha * t),
  ),
)

While #886 / #977 successfully removed the enclosing Opacity widget (avoiding full-screen saveLayer offscreen compositing), animating sigmaX and sigmaY linearly from 0 to 8 across 24 frames (@ 120 Hz) forces the rasterizer (Skia / Impeller) to regenerate and execute a full-resolution Gaussian blur shader pass on every frame.

Code Location

• lib/shared/widgets/app_dialog.dart (lines 152–159)

UI Kit Relationship

• Querya UI Kit Component: showAppDialog is the standard modal backdrop primitive (docs/ui-kit.md), used by QueryaModalDialog, QueryaConfirmDialog, CommandPaletteDialog, QuickSwitcherDialog, and all connection forms.
• Follow-up to closed perf(dialogs): eliminate BackdropFilter inside AnimatedOpacity in BlurredDialogScaffold #886. Open issue refactor(ui): migrate dialogs and forms to QueryaButton and QueryaDialogActions #1338 migrates dialog footers and action buttons to QueryaDialogActions, but does not touch the backdrop rendering.

Impact on 120 Hz Frame Budget

On 120 Hz displays (8.3 ms frame budget), full-screen blur with varying sigma frequently causes raster thread spikes (10–18 ms), dropping frames on modal open/close on Linux (X11/Wayland) and macOS.

Proposed Solution

1. Use a constant blur sigma (sigma: 8) whenever t > 0 and animate only the background tint alpha / overlay fade:

final t = progress.clamp(0.0, 1.0);
if (t <= 0) return const SizedBox.expand();
return ClipRect(
  child: BackdropFilter(
    filter: ImageFilter.blur(sigmaX: sigma, sigmaY: sigma),
    child: ColoredBox(
      color: const Color(0xFF000000).withValues(alpha: tintAlpha * t),
    ),
  ),
);

2. When Motion is Off / Reduced, snap directly to full blur or skip the filter entirely.

Acceptance Criteria

• Raster thread time during dialog enter/exit stays under 4.0 ms in DevTools Performance timeline @ 120 Hz.
• No visual pop-in or harsh transition when opening any modal.