Add game selection screen and new run wizard

Build a 3-step wizard at /runs/new: select game (themed gradient cards
grouped by generation with filter tabs), configure rules (reuses existing
RulesConfiguration), and name/create run. Remove standalone Games and Rules
pages since they're now integrated into the wizard flow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/index.ts

@@ -1,3 +1,6 @@
+export { GameCard } from './GameCard'
+export { GameGrid } from './GameGrid'
 export { Layout } from './Layout'
 export { RuleToggle } from './RuleToggle'
 export { RulesConfiguration } from './RulesConfiguration'
+export { StepIndicator } from './StepIndicator'