feat(studio): award-tier visual polish

Brightness-aware elevation tokens (FaiElevation low/medium) applied
to cards, sheets and the sidebar active item; gentle hover-lift on
Store + Doc cards; a longer, eased sidebar expand using the
under-used FaiMotion.slow; and a branded empty state carrying the
F∆I delta mark with warmer spacing. Subtle, Linear/Raycast-style —
both light and dark themes tuned.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/theme/tokens.dart

@@ -61,6 +61,50 @@ class FaiRadius {
   static const md = 10.0;
 }
 
+/// Elevation tokens. Subtle, Linear/Raycast-style depth — never
+/// heavy drop-shadows. Two resting levels plus a hover bump.
+/// Brightness-aware: dark themes lean on deeper, lower-alpha
+/// shadows (they read as ambient occlusion against near-black
+/// surfaces); light themes use softer, slightly larger spreads.
+class FaiElevation {
+  FaiElevation._();
+
+  /// Resting elevation for cards and sheets. Just enough to lift
+  /// a surface off the canvas without announcing itself.
+  static List<BoxShadow> low(Brightness b) => b == Brightness.dark
+      ? const [
+          BoxShadow(
+            color: Color(0x33000000),
+            blurRadius: 6,
+            offset: Offset(0, 2),
+          ),
+        ]
+      : const [
+          BoxShadow(
+            color: Color(0x14000000),
+            blurRadius: 8,
+            offset: Offset(0, 2),
+          ),
+        ];
+
+  /// Raised elevation for hover-lifted cards and floating sheets.
+  static List<BoxShadow> medium(Brightness b) => b == Brightness.dark
+      ? const [
+          BoxShadow(
+            color: Color(0x4D000000),
+            blurRadius: 16,
+            offset: Offset(0, 6),
+          ),
+        ]
+      : const [
+          BoxShadow(
+            color: Color(0x1F000000),
+            blurRadius: 20,
+            offset: Offset(0, 8),
+          ),
+        ];
+}
+
 /// Animation durations. Keep them under 300ms for power-user feel.
 class FaiMotion {
   FaiMotion._();