style: redesign Spielplan — date grouping, compact stats, less glossy

- Replace 3 large stat tiles with compact "2 von 104 getippt" line
- Remove phase dropdown (not useful for daily tipping)
- Group matches by actual date (Mi, 11. Juni / Do, 12. Juni)
  instead of generic "Demnächst"
- First 3 date sections open by default
- Reduce TipModal flag glossy to match MatchCard flags
- Past matches in own collapsed section

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

frontend/src/components/TipModal.module.css

@@ -77,12 +77,9 @@
 .flagLarge {
   width: 72px;
   height: 72px;
-  border-radius: 18px;
+  border-radius: 16px;
   background: var(--surface-high);
-  box-shadow:
-    var(--shadow-card),
-    inset 0 1px 0 rgba(255,255,255,0.55),
-    inset 0 -1px 0 rgba(0,0,0,0.06);
+  box-shadow: 0 2px 8px rgba(0,0,0,0.12);
   border: 1px solid var(--border-subtle);
   display: flex;
   align-items: center;
@@ -95,8 +92,8 @@
   content: '';
   position: absolute;
   top: 0; left: 0; right: 0;
-  height: 55%;
-  background: linear-gradient(180deg, rgba(255,255,255,0.55) 0%, rgba(255,255,255,0.0) 100%);
+  height: 40%;
+  background: linear-gradient(180deg, rgba(255,255,255,0.2) 0%, transparent 100%);
   pointer-events: none;
   z-index: 1;
 }