feat: implement shared-album multi-user photo album

Backend (Go/Gin/pgx): env config, pgx pool with retries, Garage S3 client
(path-style, region garage), embedded golang-migrate migrations, JWT auth with
refresh rotation, albums CRUD, members/roles/invites with no-escalation matrix,
photo upload + async imaging/goexif WebP variants, cursor pagination, presigned
URLs, likes/comments, SSE hub (album + personal), notifications, share links.

Frontend (SvelteKit 2 + Svelte 5 + Tailwind v4): API wrapper with 401 refresh,
auth store, all screens, Uploader/PhotoGrid/Lightbox/Comments, SSE client with
reconnect + REST catch-up, three themes + system via data-theme.

Deploy without Docker: Makefile builds the frontend into backend/static and
embeds it (go:embed) into a single statically linked binary; systemd unit and
nginx SSE example included.

Use gen2brain/webp (-tags nodynamic) instead of cgo kolesa-team/go-webp to keep
CGO_ENABLED=0 static builds; design.md/proposal.md updated accordingly.

frontend/src/lib/components/ThemeSwitcher.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+	import { theme, type Theme } from '$lib/stores/theme.svelte';
+
+	const options: { value: Theme; label: string; icon: string }[] = [
+		{ value: 'light', label: 'Light', icon: '☀️' },
+		{ value: 'dark', label: 'Dark', icon: '🌙' },
+		{ value: 'neutral', label: 'Neutral', icon: '🌾' },
+		{ value: 'system', label: 'System', icon: '💻' }
+	];
+</script>
+
+<div class="border-border bg-bg-elevated inline-flex gap-1 rounded-lg border p-1" role="group" aria-label="Theme">
+	{#each options as option (option.value)}
+		<button
+			type="button"
+			title={option.label}
+			aria-label={option.label}
+			aria-pressed={theme.current === option.value}
+			onclick={() => theme.set(option.value)}
+			class="rounded-md px-2 py-1 text-sm transition-colors {theme.current === option.value
+				? 'bg-primary text-primary-fg'
+				: 'text-text-muted hover:bg-bg-muted'}"
+		>
+			{option.icon}
+		</button>
+	{/each}
+</div>