feat: add toggleable hotkey hints to sidebar buttons and refac ShortcutsModal

- Add src/lib/shortcuts.ts as the single source of truth for every shortcut
- Create HotkeyHint.svelte to show OS-aware key combos (⌘ on Mac, Ctrl elsewhere)
- Make sidebar “New Chat” and “Search” buttons display their shortcuts on hover
- Add user setting to toggle these sidebar hints
- Refactor ShortcutsModal into categorized (Global, Chat, Message, Input), data-driven sections
- Introduce ShortcutItem.svelte to render each row, dividers, and multi-line text
- Fix “Focus text area” action and include “Close modal” shortcut
- Wire everything through +layout.svelte and the shortcuts registry

src/lib/components/common/HotkeyHint.svelte

@@ -0,0 +1,41 @@
+<script lang="ts">
+	import { onMount } from 'svelte';
+	import { shortcuts } from '$lib/shortcuts';
+	import { settings } from '$lib/stores';
+
+	export let name: string;
+	export let className = '';
+
+	let isMac = false;
+	let mounted = false;
+	let keys: string[] = [];
+	let isVisible = true;
+
+	onMount(() => {
+		isMac = /Mac/i.test(navigator.userAgent);
+		keys = shortcuts[name]?.keys ?? [];
+		mounted = true;
+	});
+
+	$: if (name === 'newChat' || name === 'search') {
+		isVisible = $settings?.showSidebarHotkeyHints !== false;
+	}
+
+	function formatKey(key: string): string {
+		const lowerKey = key.toLowerCase();
+
+		if (lowerKey === 'mod') return isMac ? '⌘' : 'Ctrl';
+		if (lowerKey === 'shift') return isMac ? '⇧' : 'Shift';
+		if (lowerKey.startsWith('key')) return key.slice(-1);
+
+		return key;
+	}
+</script>
+
+{#if mounted && isVisible}
+	<div
+		class="hidden md:flex items-center self-center text-xs text-gray-400 dark:text-gray-600 {className}"
+	>
+		<span>{keys.map(formatKey).join(isMac ? '' : '+')}</span>
+	</div>
+{/if}