Add workspace management foundation

internal/app/static/style.css

@@ -47,6 +47,9 @@ h2 { margin-bottom: 8px; font-size: 1.25rem; letter-spacing: -.025em; }
 .brand small { display: block; margin-top: 1px; color: var(--muted); font-size: .62rem; font-weight: 500; letter-spacing: .02em; }
 .brand-mark { display: grid; width: 35px; height: 35px; place-items: center; border-radius: 11px; background: var(--green); color: #fff; font-family: Vazirmatn, Tahoma, sans-serif; font-size: 1.25rem; }
 .sidebar nav { margin-top: 48px; }
+.workspace-switcher { margin: 0 0 12px; }
+.workspace-switcher label { display:block; color:var(--muted); font-size:.72rem; font-weight:700; text-transform:uppercase; letter-spacing:.08em; }
+.workspace-switcher select { width:100%; margin-top:5px; border:1px solid var(--line); border-radius:10px; background:var(--surface); color:var(--ink); padding:8px 9px; font:inherit; }
 .sidebar nav a { display: flex; align-items: center; gap: 11px; min-height: 43px; margin: 4px 0; padding: 0 12px; border-radius: 10px; color: #53605a; font-size: .87rem; font-weight: 600; }
 .sidebar nav a:hover { background: #eef2ef; color: var(--ink); }
 .sidebar nav a.active { background: var(--mint); color: var(--green); }