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/ferese.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" role="img" aria-label="Ferese symbol: tiled F">
+  <rect x="20" y="20" width="50" height="160" rx="8" fill="currentColor"/>
+  <rect x="80" y="20" width="100" height="55" rx="8" fill="currentColor"/>
+  <rect x="80" y="95" width="70" height="45" rx="8" fill="currentColor"/>
+</svg>