feat(shared): add DevBuildBadge component and getBuildDefines() helper

- shared-vite-config: getBuildDefines() injects __BUILD_HASH__ and
  __BUILD_TIME__ as compile-time constants via Vite define
- shared-ui: DevBuildBadge component shows git hash + build timestamp
  in a small fixed badge at bottom-right (click to expand)
- Integrated into mukke-web for deployment verification

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-ui/src/components/DevBuildBadge.svelte

@@ -0,0 +1,77 @@
+<script lang="ts">
+	interface Props {
+		commitHash?: string;
+		buildTime?: string;
+	}
+
+	let { commitHash = 'unknown', buildTime = '' }: Props = $props();
+
+	let formattedTime = $derived.by(() => {
+		if (!buildTime) return '';
+		try {
+			const d = new Date(buildTime);
+			return d.toLocaleString('de-DE', {
+				month: '2-digit',
+				day: '2-digit',
+				hour: '2-digit',
+				minute: '2-digit',
+			});
+		} catch {
+			return buildTime;
+		}
+	});
+
+	let expanded = $state(false);
+</script>
+
+<button
+	class="dev-badge"
+	class:expanded
+	onclick={() => (expanded = !expanded)}
+	title="Build: {commitHash} | {buildTime}"
+>
+	{#if expanded}
+		<span class="dev-badge-detail">{commitHash} &middot; {formattedTime}</span>
+	{:else}
+		<span class="dev-badge-hash">{commitHash}</span>
+	{/if}
+</button>
+
+<style>
+	.dev-badge {
+		position: fixed;
+		bottom: 8px;
+		right: 8px;
+		z-index: 9999;
+		font-family: ui-monospace, monospace;
+		font-size: 10px;
+		line-height: 1;
+		padding: 3px 6px;
+		border-radius: 4px;
+		border: none;
+		background: rgba(0, 0, 0, 0.4);
+		color: rgba(255, 255, 255, 0.7);
+		cursor: pointer;
+		pointer-events: auto;
+		opacity: 0.5;
+		transition: opacity 150ms ease;
+		backdrop-filter: blur(4px);
+		-webkit-backdrop-filter: blur(4px);
+	}
+
+	.dev-badge:hover {
+		opacity: 1;
+	}
+
+	.dev-badge.expanded {
+		opacity: 0.85;
+	}
+
+	.dev-badge-hash {
+		letter-spacing: 0.5px;
+	}
+
+	.dev-badge-detail {
+		white-space: nowrap;
+	}
+</style>