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/routes/+layout.ts

@@ -0,0 +1,5 @@
+// Tauri doesn't have a Node.js server to do proper SSR
+// so we use adapter-static with a fallback to index.html to put the site in SPA mode
+// See: https://svelte.dev/docs/kit/single-page-apps
+// See: https://v2.tauri.app/start/frontend/sveltekit/ for more info
+export const ssr = false;

frontend/src/routes/+page.svelte

@@ -0,0 +1,61 @@
+<script lang="ts">
+	import { onMount } from "svelte";
+	import Sidebar from "$lib/components/Sidebar.svelte";
+	import Placeholder from "$lib/components/Placeholder.svelte";
+	import { DEFAULT_PAGE } from "$lib/sidebar";
+	import { initTheme } from "$lib/theme";
+	import { VIEWS } from "$lib/views/registry";
+
+	let activePage = $state(DEFAULT_PAGE);
+	let ActiveView = $derived(VIEWS[activePage]);
+
+	onMount(() => {
+		initTheme();
+	});
+</script>
+
+<div class="shell">
+	<Sidebar bind:activePage />
+	<main class="content">
+		{#if ActiveView}
+			<ActiveView />
+		{:else}
+			<Placeholder page={activePage} />
+		{/if}
+	</main>
+</div>
+
+<style>
+	:global(*) {
+		box-sizing: border-box;
+	}
+
+	:global(html, body) {
+		margin: 0;
+		border: none;
+		outline: none;
+		height: 100%;
+		color-scheme: dark;
+		background-color: var(--bg, #0c0c0c);
+		color: var(--fg);
+		font-family: var(--font-family, sans-serif);
+		font-size: var(--font-size-base, 14px);
+	}
+
+	:global(#svelte) {
+		height: 100%;
+	}
+
+	.shell {
+		display: flex;
+		height: 100vh;
+		background-color: var(--bg, #0c0c0c);
+	}
+
+	.content {
+		flex: 1;
+		min-width: 0;
+		overflow-y: auto;
+		overscroll-behavior: contain;
+	}
+</style>