Share native theme components and fix dialog backdrop blur

Centralize native palettes, presets, typography, controls, menus, calendar rendering, and icon assets in ferese-theme. Reuse the shared components across the shell, settings, lock screen, authentication prompt, and screen-sharing chooser.

Attach themed dialogs to compositor-owned materials, correct backdrop capture and sampling across output transforms, smooth patterned backgrounds, and prevent clipped picker buttons with the configured shell font.

crates/ferese-theme/assets/icons/wifi.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M2.5 7.8a14.5 14.5 0 0 1 19 0"/>
+  <path d="M5.7 11.4a9.6 9.6 0 0 1 12.6 0"/>
+  <path d="M8.9 15a4.7 4.7 0 0 1 6.2 0"/>
+  <circle cx="12" cy="18.4" r="1.2" fill="currentColor" stroke="none"/>
+</svg>