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/Switch.svelte

@@ -0,0 +1,43 @@
+<script lang="ts">
+	let { value = $bindable(), ariaLabel }: { value: boolean; ariaLabel?: string } = $props();
+</script>
+
+<button
+	type="button"
+	class="switch"
+	class:on={value}
+	role="switch"
+	aria-checked={value}
+	aria-label={ariaLabel}
+	onclick={() => (value = !value)}
+>
+	<span class="knob"></span>
+</button>
+
+<style>
+	.switch {
+		width: 40px;
+		height: 22px;
+		flex-shrink: 0;
+		border-radius: 999px;
+		border: none;
+		background-color: var(--overlay);
+		padding: 2px;
+		cursor: pointer;
+		display: flex;
+		align-items: center;
+	}
+
+	.switch.on {
+		background-color: var(--accent);
+		justify-content: flex-end;
+	}
+
+	.knob {
+		width: 18px;
+		height: 18px;
+		border-radius: 50%;
+		background-color: var(--on-surface);
+		display: block;
+	}
+</style>