fix(web): add keyboard access to charts

[xingkaixin]

Summary

• make the timeline minimap scrollbar operable with Arrow, Page, Home, and End keys
• expose Daily Activity and Daily Token bars to focus, pointer, and touch interaction
• add accessible names and screen-reader data tables for chart buckets
• expose model token values alongside percentages and hide the redundant pie graphic
• add timeline, Dashboard, and Playwright accessibility coverage

Root cause

The minimap declared ARIA scrollbar semantics without keyboard behavior or a tab stop. Dashboard
bars only reacted to mouse hover and exposed details through title, leaving keyboard, touch, and
screen-reader users without an equivalent path to the data.

Impact

Every chart now has a labeled region and complete textual data equivalent. Interactive bars share
the same detail state across focus, pointer, and tap, while the timeline scrollbar implements the
standard keyboard navigation model.

Validation

• pnpm test:e2e
• pnpm --filter @codesesh/web test
• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test

Issue: CS-40