feat: polish theme tooling, diagnostics and launcher workflows

Expand the theme editor with multi-select, snapping, context menus, draft
recovery and export validation; add theme backups and trust prompts. Add
in-app diagnostics and rotating log bundle viewing, hardware-accelerated
theme video playback on macOS, and refresh the settings, setup wizard and
theme gallery layouts.

src/lib/components/ui/context-menu/context-menu-sub.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+  import { ContextMenu as ContextMenuPrimitive } from 'bits-ui';
+
+  let { open = $bindable(false), ...restProps }: ContextMenuPrimitive.SubProps = $props();
+</script>
+
+<ContextMenuPrimitive.Sub bind:open {...restProps} />