Implement fixed viewport with internal scrolling

Restructure app layout to prevent whole-page scrolling. The viewport
is now locked at 100vh with overflow:hidden, and individual content
areas scroll internally.

Architecture changes:
- Lock html/body at 100% height with overflow:hidden
- Fix Layout component to 100vh, Screen to 100% height
- Enable internal scrolling for content areas with flex:1 + overflow-y:auto

New game wizard improvements:
- Split forms into three sections: form-header (fixed), form-content
  (scrollable), form-footer (fixed with arrow navigation)
- Fixes issue where many player names pushed navigation arrows off-screen
- Applied to Player1Step, Player2Step, Player3Step, GameTypeStep

Game list improvements:
- Filter buttons stay fixed at top
- Games container scrolls internally with overflow-y:auto
- "Neues Spiel" button wrapped with flex-shrink:0

Game detail improvements:
- Game controls stay visible while content scrolls

Additional changes:
- Add Playwright test artifact exclusions to .gitignore
- Add Docker build instructions to README.md
- Remove unnecessary setSelectionRange calls from player input steps

Benefits:
- No accidental page scrolling
- Cleaner mobile UX (no address bar show/hide issues)
- Navigation controls always visible
- Predictable, contained scrolling behavior

src/components/NewGame.module.css

@@ -13,10 +13,12 @@
 .screen-content {
     display: flex;
     flex-direction: column;
-    min-height: 100vh;
+    height: 100%;
     padding: var(--space-lg);
     overflow-y: auto;
+    overflow-x: hidden;
     -webkit-overflow-scrolling: touch;
+    min-height: 0;
 }
 .screen-title {
     font-size: var(--font-size-xxl);
@@ -122,11 +124,31 @@
     background: var(--color-surface);
     border-radius: var(--radius-xl);
     box-shadow: var(--shadow-lg);
-    padding: var(--space-xl) var(--space-lg) var(--space-lg) var(--space-lg);
+    padding: 0;
     display: flex;
     flex-direction: column;
-    gap: var(--space-lg);
     border: 1px solid var(--color-border);
+    flex: 1;
+    min-height: 0;
+    overflow: hidden;
+}
+
+.form-header {
+    flex-shrink: 0;
+    padding: var(--space-xl) var(--space-lg) var(--space-md) var(--space-lg);
+}
+
+.form-content {
+    flex: 1;
+    overflow-y: auto;
+    overflow-x: hidden;
+    padding: 0 var(--space-lg);
+    min-height: 0;
+}
+
+.form-footer {
+    flex-shrink: 0;
+    padding: var(--space-lg);
 }
 .progress-indicator {
     display: flex;