test: replace stub tests with real coverage, add E2E for new features
Replace 3 stub test files that were testing CSS constants and inline
calculations with meaningful tests against real exported functions:
- sheets-ux-improvements: cell-styles.ts borders/wrap/striped + sort.ts
- ux-iteration-3: formulas.ts parseRef/colToLetter/formatCell + row-col-ops.ts
- landing-overhaul: all 20+ functions from landing-utils.ts
Add new unit tests for blob-upload.ts (13 tests covering upload/download/
list/delete/readFileAsBuffer/blobToObjectUrl with proper FileReader mock).
Add E2E tests for slides, diagrams, and forms features with Playwright
helpers (createNewSlides, createNewDiagram, createNewForm).
Resolves #296, #297, #298, #299, #300, #301