feat(frontend): global keyboard navigation and shortcuts help overlay

Adds a single window-level key dispatcher in the root layout: `g` then
c/t/f/p/s (or the digits 1–5) jump between the five sections, honouring
each section's remembered URL so you land back on the same filter and
scroll. `?` toggles a shortcuts cheat-sheet overlay. The handler stays
out of the way while typing in inputs or when a browser/OS modifier is
held. This is the foundation for the per-context keymaps (grid, viewer,
tag/filter pickers) that follow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/components/layout/KeyboardHelp.svelte

@@ -0,0 +1,192 @@
+<script lang="ts">
+	interface Props {
+		onClose: () => void;
+	}
+
+	let { onClose }: Props = $props();
+
+	// Static cheat-sheet of the app's shortcuts, grouped by context. Kept in sync
+	// by hand with the per-context handlers (global nav here, the rest on the
+	// Files page / viewer / tag pickers).
+	const groups: { title: string; rows: [string, string][] }[] = [
+		{
+			title: 'Anywhere',
+			rows: [
+				['g then c / t / f / p / s', 'Go to Categories / Tags / Files / Pools / Settings'],
+				['1 – 5', 'Jump to a section'],
+				['?', 'Toggle this help'],
+				['/', 'Focus the filter / search']
+			]
+		},
+		{
+			title: 'File grid',
+			rows: [
+				['↑ ↓ ← →', 'Move focus between files'],
+				['Enter', 'Open the focused file'],
+				['Space / x', 'Select / deselect'],
+				['e', 'Edit tags (focus the tag filter)'],
+				['p', 'Add to pool'],
+				['Del', 'Move to trash'],
+				['Esc', 'Clear selection']
+			]
+		},
+		{
+			title: 'Viewer',
+			rows: [
+				['← / → or j / k', 'Previous / next file'],
+				['e', 'Jump to tags & focus the filter'],
+				['Esc', 'Close']
+			]
+		},
+		{
+			title: 'Tag editor / filter',
+			rows: [
+				['↓ ↑', 'Highlight a suggestion'],
+				['Enter', 'Add the highlighted tag'],
+				['← →', 'Move across added tags / tokens (empty input)'],
+				['Del', 'Remove the focused tag / token'],
+				['& | ! ( )', 'Insert an operator (filter only)'],
+				['Ctrl+Enter', 'Apply the filter'],
+				['Ctrl+Backspace', 'Reset the filter'],
+				['Esc', 'Close']
+			]
+		}
+	];
+</script>
+
+<!-- svelte-ignore a11y_click_events_have_key_events a11y_no_static_element_interactions -->
+<div class="backdrop" role="presentation" onclick={onClose}></div>
+<div class="sheet" role="dialog" aria-label="Keyboard shortcuts" aria-modal="true">
+	<div class="head">
+		<span class="title">Keyboard shortcuts</span>
+		<button class="close" onclick={onClose} aria-label="Close">
+			<svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+				<path
+					d="M3 3l10 10M13 3L3 13"
+					stroke="currentColor"
+					stroke-width="1.8"
+					stroke-linecap="round"
+				/>
+			</svg>
+		</button>
+	</div>
+	<div class="body">
+		{#each groups as group}
+			<section class="group">
+				<h3 class="group-title">{group.title}</h3>
+				{#each group.rows as [keys, desc]}
+					<div class="row">
+						<kbd class="keys">{keys}</kbd>
+						<span class="desc">{desc}</span>
+					</div>
+				{/each}
+			</section>
+		{/each}
+	</div>
+</div>
+
+<style>
+	.backdrop {
+		position: fixed;
+		inset: 0;
+		z-index: 300;
+		background: rgba(0, 0, 0, 0.5);
+	}
+
+	.sheet {
+		position: fixed;
+		left: 50%;
+		top: 50%;
+		transform: translate(-50%, -50%);
+		z-index: 301;
+		width: min(560px, calc(100vw - 24px));
+		max-height: min(80dvh, 640px);
+		display: flex;
+		flex-direction: column;
+		background-color: var(--color-bg-secondary);
+		border-radius: 14px;
+		box-shadow: 0 8px 40px rgba(0, 0, 0, 0.5);
+		animation: pop 0.16s ease-out;
+	}
+
+	@keyframes pop {
+		from {
+			transform: translate(-50%, -48%) scale(0.98);
+			opacity: 0;
+		}
+		to {
+			transform: translate(-50%, -50%) scale(1);
+			opacity: 1;
+		}
+	}
+
+	.head {
+		display: flex;
+		align-items: center;
+		padding: 14px 16px 10px;
+	}
+
+	.title {
+		flex: 1;
+		font-size: 1rem;
+		font-weight: 600;
+	}
+
+	.close {
+		background: none;
+		border: none;
+		cursor: pointer;
+		color: var(--color-text-muted);
+		padding: 4px;
+		display: flex;
+	}
+
+	.close:hover {
+		color: var(--color-text-primary);
+	}
+
+	.body {
+		padding: 0 16px 18px;
+		overflow-y: auto;
+		display: grid;
+		grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
+		gap: 6px 20px;
+	}
+
+	.group {
+		break-inside: avoid;
+		padding-top: 8px;
+	}
+
+	.group-title {
+		font-size: 0.72rem;
+		text-transform: uppercase;
+		letter-spacing: 0.06em;
+		color: var(--color-accent);
+		margin: 0 0 6px;
+	}
+
+	.row {
+		display: flex;
+		align-items: baseline;
+		gap: 10px;
+		padding: 3px 0;
+	}
+
+	.keys {
+		flex-shrink: 0;
+		font-family: var(--font-sans);
+		font-size: 0.72rem;
+		color: var(--color-text-primary);
+		background-color: var(--color-bg-elevated);
+		border: 1px solid color-mix(in srgb, var(--color-accent) 25%, transparent);
+		border-radius: 5px;
+		padding: 2px 6px;
+		white-space: nowrap;
+	}
+
+	.desc {
+		font-size: 0.82rem;
+		color: var(--color-text-muted);
+	}
+</style>

frontend/src/routes/+layout.svelte

@@ -1,8 +1,9 @@
 <script lang="ts">
 	import '../app.css';
 	import { page } from '$app/stores';
-	import { afterNavigate } from '$app/navigation';
+	import { afterNavigate, goto } from '$app/navigation';
 	import { themeStore, toggleTheme } from '$lib/stores/theme';
+	import KeyboardHelp from '$lib/components/layout/KeyboardHelp.svelte';
 
 	let { children } = $props();
 
@@ -51,10 +52,84 @@
 		const item = navItems.find((it) => it.match === url.pathname);
 		if (item) lastUrl[item.match] = url.pathname + url.search;
 	});
+
+	// ---- Global keyboard navigation -----------------------------------------
+	let helpOpen = $state(false);
+
+	// g-then-letter and 1–5 jump between sections; both honour the remembered
+	// per-section URL so you land back on the same filter/scroll.
+	const G_MAP: Record<string, string> = {
+		c: '/categories',
+		t: '/tags',
+		f: '/files',
+		p: '/pools',
+		s: '/settings'
+	};
+	const NUM_MAP = navItems.map((it) => it.match); // 1→categories … 5→settings
+
+	let pendingG = false;
+	let gTimer: ReturnType<typeof setTimeout> | undefined;
+
+	function go(match: string) {
+		goto(lastUrl[match] ?? match);
+	}
+
+	function isEditable(t: EventTarget | null): boolean {
+		return (
+			t instanceof HTMLElement &&
+			(t.isContentEditable || ['INPUT', 'TEXTAREA', 'SELECT'].includes(t.tagName))
+		);
+	}
+
+	function onGlobalKey(e: KeyboardEvent) {
+		if (helpOpen && e.key === 'Escape') {
+			helpOpen = false;
+			return;
+		}
+		// Stay out of the way while typing or when a browser/OS combo is held.
+		if (isEditable(e.target) || e.metaKey || e.ctrlKey || e.altKey) return;
+		if (isLogin) return;
+
+		if (e.key === '?') {
+			helpOpen = !helpOpen;
+			e.preventDefault();
+			return;
+		}
+
+		if (pendingG) {
+			pendingG = false;
+			clearTimeout(gTimer);
+			const dest = G_MAP[e.key.toLowerCase()];
+			if (dest) {
+				e.preventDefault();
+				go(dest);
+			}
+			return;
+		}
+		if (e.key === 'g') {
+			pendingG = true;
+			clearTimeout(gTimer);
+			gTimer = setTimeout(() => (pendingG = false), 1000);
+			return;
+		}
+		if (e.key >= '1' && e.key <= '5') {
+			const dest = NUM_MAP[Number(e.key) - 1];
+			if (dest) {
+				e.preventDefault();
+				go(dest);
+			}
+		}
+	}
 </script>
 
+<svelte:window onkeydown={onGlobalKey} />
+
 {@render children()}
 
+{#if helpOpen}
+	<KeyboardHelp onClose={() => (helpOpen = false)} />
+{/if}
+
 {#if !isLogin && !isAdmin}
 	<footer>
 		{#each navItems as item}