feat: game search / filter (#35)

[yanekyuk]

Summary

• Adds live fuzzy-filter to the Main screen games list (/ to enter, Esc to clear)
• Filter uses SkimMatcherV2 (case-insensitive, subsequence match) via fuzzy-matcher crate
• Filter state persists across screen transitions; selection is preserved on Esc (translates filtered index back to full-list position)

Changes

• App: filter_query, filter_mode fields + filtered_games() method; selected_game() and scroll_down() are now filter-aware
• handle_main: branches on filter_mode — action keys blocked while filtering, ↑/↓ still navigate, Esc exits cleanly
• draw_game_list: filter row (/ query▌) shown at top of GAMES block when active; list built from filtered_games()
• Status bar swaps to filter hints when in filter mode
• 8 unit tests added covering filter logic, selection, and scroll bounds

Test Plan

• Press / — filter row appears, status bar changes to filter hints
• Type partial name — list narrows in real time
• Try fuzzy input (e.g. cs2 for Counter-Strike 2) — matches correctly
• Press ↑/↓ — navigates filtered list; j/k type into filter
• Navigate to a game via filter, press Esc — filter clears, full list restored, same game still selected
• Navigate to another screen while filter is active, return — filter still applied
• cargo test — all 63 tests pass

Implements #35