feat(desktop): implement workspace foundation + desktop-client skeleton

Plan 0 (workspaces): add workspaces + workspace_memberships schema, extend
JWT/Actor/claims with primary_workspace_id, seed default workspace per tenant,
thread workspace_id through tenant monitoring quota.

Plan A (desktop skeleton): new Electron app (apps/desktop-client) with main/
preload/renderer, shared Vue component package (packages/ui-shared), and server
surface — desktop client registration + token rotation + heartbeat, SSE task
event stream, desktop accounts/tasks/content handlers, publish job endpoint,
and supporting repositories, services, sqlc queries, and migrations.

Hard cutover per plan: remove browser-extension monitoring callback endpoints,
stub legacy media API in admin-web, and delete monitoring_callback_handler.go.

apps/desktop-client/src/renderer/components/MetricCard.vue

@@ -0,0 +1,75 @@
+<script setup lang="ts">
+defineProps<{
+  eyebrow: string;
+  value: string | number;
+  label: string;
+  tone?: "neutral" | "brand" | "success" | "warn" | "danger" | "info";
+}>();
+</script>
+
+<template>
+  <article class="metric-card" :class="tone ?? 'neutral'">
+    <p class="eyebrow">{{ eyebrow }}</p>
+    <strong class="value">{{ value }}</strong>
+    <p class="label">{{ label }}</p>
+  </article>
+</template>
+
+<style scoped>
+.metric-card {
+  padding: 20px;
+  border-radius: 12px;
+  border: 1px solid #e6edf5;
+  background: #ffffff;
+  display: flex;
+  flex-direction: column;
+}
+
+.metric-card.brand {
+  border-top: 3px solid #1677ff;
+}
+
+.metric-card.success {
+  border-top: 3px solid #52c41a;
+}
+
+.metric-card.warn {
+  border-top: 3px solid #faad14;
+}
+
+.metric-card.danger {
+  border-top: 3px solid #ff4d4f;
+}
+
+.metric-card.info {
+  border-top: 3px solid #13c2c2;
+}
+
+.eyebrow,
+.label {
+  margin: 0;
+}
+
+.eyebrow {
+  color: #8c8c8c;
+  font-size: 11px;
+  letter-spacing: 0.1em;
+  text-transform: uppercase;
+}
+
+.value {
+  display: block;
+  margin-top: 12px;
+  font-size: 28px;
+  font-weight: 600;
+  color: #1a1a1a;
+  line-height: 1;
+}
+
+.label {
+  margin-top: 8px;
+  color: #8c8c8c;
+  font-size: 12px;
+  line-height: 1.5;
+}
+</style>