Migrate bos-settings from GTK4 to Tauri + Svelte 5

Ports every remaining view (Keybinds, the last one — dual Flat/MultiLayout
schema detection, per-action dedicated fields with a raw-JSON fallback for
anything not modeled) and removes the now-fully-dead GTK4 crate (root
Cargo.toml + src/), leaving src-tauri/ as the single Rust binary crate.

Also folds in the UX pass done alongside the migration: responsive
multi-column layout instead of a single centered column, chip/dropdown
pickers replacing free-typed fields (Wi-Fi networks, Bar style, keyboard
layout, tag lists), consistent pill-switch toggles and boxed-list card
styling across every page, and a sidebar scroll-chaining fix.

frontend/src/lib/components/Hint.svelte

@@ -0,0 +1,18 @@
+<script lang="ts">
+	let { text }: { text: string } = $props();
+</script>
+
+<p class="hint">{text}</p>
+
+<style>
+	.hint {
+		opacity: 0.6;
+		font-size: var(--font-size-secondary, 12px);
+		margin: 0 0 var(--space-xs, 4px);
+		/* No-op unless this Hint is a direct child of ViewScaffold's CSS
+		   grid (i.e. used standalone between Groups, not nested inside
+		   one) — in that case it spans full width instead of getting
+		   squeezed into whichever grid column it happened to land in. */
+		grid-column: 1 / -1;
+	}
+</style>