Refactor: extract shared CSS, split reducer, clean up dead code

[lchoward]

Summary

• Delete dead GameHistory component and unused react.svg asset
• Extract duplicated CSS into shared.css (scoreboard styles, bust-chance colors, CPU badges, tiebreaker/cheater toggles)
• Create reusable BustChanceBanner component replacing duplicate IIFE blocks in PlayRound and RoundEntry
• Extract flattenHandWithCancelled utility into handUtils.js and createEmptyDealtCards into deckUtils.js
• Split play-mode actions (~450 lines) into separate playModeReducer.js
• Remove global SET_DIALOG action in favor of local useState in HomeScreen and GameScreen
• Standardize NAVIGATE payload to { screen, editingRound?, playerId? } across all components
• Remove unused getDealtCards/getDeckStatus helpers and reorganize test utils

Net result: -426 lines (1000 added, 1426 removed), with cleaner separation of concerns and less duplication.

Fixes FLI-9

Test plan

• npm run build passes
• All 140 tests pass (npx vitest run)
• Lint shows only pre-existing warnings
• Manual smoke test: Score Tracker mode — start game, enter round, verify scoreboard
• Manual smoke test: Play Mode — start game, play round with cheater mode, verify bust-chance banner
• Manual smoke test: tiebreaker banner appears when players tie at 200+
• Manual smoke test: CPU players behave correctly

🤖 Generated with Claude Code

[linear]

FLI-9 see if the codebase should be re-factored