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/styles/index.css

@@ -85,11 +85,15 @@
     }
 }
 
+html, body {
+    height: 100%;
+    overflow: hidden;
+}
+
 body {
     font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
     background-color: var(--color-background);
     color: var(--color-text);
-    min-height: 100vh;
     overscroll-behavior: none;
     line-height: 1.5;
     -webkit-font-smoothing: antialiased;