refactor(sheets): decompose formulas.ts and mouse-events.ts into focused modules
Extract tokenizer, parser, resize, autofit, and fill logic into dedicated files:
- formula-tokenizer.ts: lexical analysis (TokenType, tokenize)
- formula-parser.ts: recursive descent Parser class
- resize-handlers.ts: column/row resize with visual guide
- autofit-handlers.ts: auto-fit column width and row height
- fill-handlers.ts: drag-to-fill logic and preview visuals
formulas.ts: 794 → 195 lines (keeps function dispatch, evaluate, extractRefs, formatCell)
mouse-events.ts: 548 → 261 lines (keeps event routing and cell selection)