Add Letter Triangles game with seeded generation and tests

[ianfhunter]

Motivation

• Add a new puzzle type (Letter Triangles) to the Word Games pack that provides a reproducible, seed-driven triangular tile-placement word puzzle.
• Ensure puzzles are generatable and testable deterministically and integrate cleanly with existing game UI patterns (SeedDisplay, GiveUpButton, GameResult).

Description

• Added a new game page and UI: src/pages/LetterTriangles/LetterTriangles.jsx with supporting stylesheet LetterTriangles.module.css and README.md documenting rules.
• Implemented puzzle generation and helper logic in src/pages/LetterTriangles/letterTrianglesLogic.js with a seeded backtracking search over dictionary word pools and a deterministic fallback layout when dictionary matching fails.
• Registered the game in the Word Games pack and routing: src/packs/word-games/manifest.js and src/App.jsx, and added an SVG icon asset src/assets/icons/letter-triangles.svg.
• Added i18n strings for English and Spanish (src/i18n/locales/en.json, src/i18n/locales/es.json) and a Vitest test file src/pages/LetterTriangles/LetterTriangles.test.js validating determinism, length invariants, line reconstruction, and solved-state detection.

Testing

• Ran npm run test:run -- src/pages/LetterTriangles/LetterTriangles.test.js and the new generator/tests passed (4 tests all green).
• Ran npm run test:run -- src/tests/gameReadmeCheck.test.js and npm run test:run -- src/tests/gameModuleImports.test.js, both of which passed to confirm README presence and module importability.
• Ran npm run lint which surfaced many pre-existing repo lint errors unrelated to this change, so lint is noted but not blocking this PR.

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