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/streaming.ts

@@ -0,0 +1,25 @@
+// Frontend half of the event-streaming command pattern (see
+// src-tauri/src/commands/streaming.rs) — runs a command, appends each
+// stdout/stderr line to a reactive log as it arrives, and resolves once the
+// process exits with whether it succeeded.
+
+import { invoke } from "@tauri-apps/api/core";
+import { listen } from "@tauri-apps/api/event";
+
+export async function runStreamingCommand(
+	program: string,
+	args: string[],
+	onLine: (line: string) => void,
+): Promise<boolean> {
+	const sessionId = crypto.randomUUID();
+
+	const unlisten = await listen<{ session_id: string; line: string }>("cmd-output", (event) => {
+		if (event.payload.session_id === sessionId) onLine(event.payload.line);
+	});
+
+	try {
+		return await invoke<boolean>("run_streaming_command", { sessionId, program, args });
+	} finally {
+		unlisten();
+	}
+}