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/ViewScaffold.svelte

@@ -0,0 +1,40 @@
+<script lang="ts">
+	import type { Snippet } from "svelte";
+
+	let { title, children }: { title: string; children: Snippet } = $props();
+</script>
+
+<div class="view">
+	<h1 class="title">{title}</h1>
+	<div class="content">
+		{@render children()}
+	</div>
+</div>
+
+<style>
+	.view {
+		padding: var(--space-xl, 20px) var(--space-xl, 20px) 48px;
+		height: 100%;
+		overflow-y: auto;
+	}
+
+	.title {
+		font-size: 1.6em;
+		font-weight: bold;
+		margin: 0 0 var(--space-xl, 20px);
+	}
+
+	/* Groups lay out as a responsive grid rather than a single narrow
+	   column — on a wide (tiled/maximized) window that means multiple
+	   independent setting groups sit side by side instead of one column
+	   with a wall of empty space either side. `.group.wide` (device
+	   lists, thumbnail grids, log views — anything that reads badly
+	   squeezed into a card) opts out via `grid-column: 1 / -1`. */
+	.content {
+		display: grid;
+		grid-template-columns: repeat(auto-fit, minmax(340px, 1fr));
+		gap: var(--space-xl, 20px) 32px;
+		align-items: start;
+		max-width: 1400px;
+	}
+</style>