feat: add interactive shadow patterns — hover lift, active tab elevation

Shadows communicate interaction state, not decoration:
- Primary buttons: cyan-tinted shadow + lift on hover
- Ghost buttons: subtle shadow + lift on hover
- Both: shadow shrinks on active/click (press effect)
- tab-active-shadow: elevation shadow for selected tabs/toggles
- card-lift: optional hover lift for interactive cards

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

DESIGN-SYSTEM.md

@@ -321,11 +321,31 @@ Color: accent-text
 
 ## Shadows
 
-Minimal to none. No decorative shadows.
+Shadows communicate **interaction state**, not decoration. Elements earn shadows through user interaction.
 
-- Cards: none (border only)
-- Dropdowns/flyouts: `0 4px 12px rgba(0,0,0,0.3)` if needed
-- Logo mark: `0 2px 8px rgba(14,165,233,0.25)` (subtle brand glow on logo only)
+**Resting state:** No shadows. Elements are flat with 1px borders.
+
+**Hover state (buttons):** Subtle shadow + lift effect. Element feels like it rises off the surface.
+- Primary button hover: `0 2px 8px rgba(34,211,238,0.25)` + `translateY(-1px)` — cyan-tinted shadow
+- Ghost button hover: `0 2px 6px rgba(0,0,0,0.2)` + `translateY(-1px)`
+- Active/click: shadow shrinks, element "presses down" to `translateY(0)`
+
+**Active/selected state (tabs, toggles):** The selected item gets an elevation shadow to feel raised above siblings.
+- Active tab: `0 2px 8px rgba(0,0,0,0.25)` — class: `tab-active-shadow`
+
+**Card hover lift (optional):** For cards that should feel interactive (clickable list items, session cards).
+- Hover: `0 4px 12px rgba(0,0,0,0.2)` + `translateY(-2px)` — class: `card-lift`
+- Active: shadow shrinks to `0 1px 4px rgba(0,0,0,0.15)`
+
+**Overlays:** Flyouts, dropdowns, modals get elevation shadows to separate from content.
+- Drawer/flyout: `4px 0 12px rgba(0,0,0,0.2)`
+- Dropdown: `0 4px 12px rgba(0,0,0,0.3)`
+- Modal: `0 8px 24px rgba(0,0,0,0.4)`
+
+**What NOT to do:**
+- No shadows on resting cards (border only)
+- No permanent decorative shadows
+- No colored glow shadows (except primary button cyan tint on hover)
 
 ---
 

frontend/src/index.css

@@ -178,9 +178,15 @@
   border-radius: 5px;
   padding: 9px 16px;
   font-size: 13px;
-  transition: filter 150ms ease;
+  transition: filter 150ms ease, box-shadow 150ms ease, transform 150ms ease;
   &:hover {
     filter: brightness(1.1);
+    box-shadow: 0 2px 8px rgba(34, 211, 238, 0.25);
+    transform: translateY(-1px);
+  }
+  &:active {
+    box-shadow: 0 1px 2px rgba(34, 211, 238, 0.15);
+    transform: translateY(0);
   }
 }
 
@@ -191,11 +197,38 @@
   border-radius: 5px;
   padding: 9px 16px;
   font-size: 13px;
-  transition: background 150ms ease, color 150ms ease, border-color 150ms ease;
+  transition: background 150ms ease, color 150ms ease, border-color 150ms ease, box-shadow 150ms ease, transform 150ms ease;
   &:hover {
     background: var(--color-bg-elevated);
     color: var(--color-text-primary);
     border-color: var(--color-border-hover);
+    box-shadow: 0 2px 6px rgba(0, 0, 0, 0.2);
+    transform: translateY(-1px);
+  }
+  &:active {
+    box-shadow: 0 1px 2px rgba(0, 0, 0, 0.15);
+    transform: translateY(0);
+  }
+}
+
+/* ── Interactive shadow patterns ────────────────── */
+
+/* Tab/toggle group: active item gets elevation shadow */
+@utility tab-active-shadow {
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.25);
+}
+
+/* Card hover lift — optional, add to cards that should feel interactive */
+@utility card-lift {
+  transition: box-shadow 200ms ease, transform 200ms ease, border-color 200ms ease;
+  &:hover {
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.2);
+    transform: translateY(-2px);
+    border-color: var(--color-border-hover);
+  }
+  &:active {
+    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.15);
+    transform: translateY(0);
   }
 }