refactor(frontend): enable checked indexed access in TypeScript

[lovasoa]

TypeScript currently treats an arbitrary array or dictionary lookup as a guaranteed value. Enable noUncheckedIndexedAccess for the frontend and both inherited test configurations so missing entries must be handled explicitly.

The chart label merge now advances narrowed iterator results instead of shifting arrays and casting away possible missing values. Chart palettes use fixed tuples, DOM structure and table sort invariants are checked, and coordinate pairs are narrowed before passing them to Leaflet. Existing assertions now report missing entries without non-null assertions or weakened types. Add coverage for all-empty series and distinct null/zero labels.

Validation:

• npm run format, npm test, and npm run build
• All 82 Playwright component fixtures passed using a SQLPage binary built with this branch’s assets.
• The changed official-site stacked-chart assertion passed.