Improve mobile dashboard layout

[ridecontrol]

What changed

• make dashboard summaries, charts, and trainer controls fit phone-width viewports
• keep the footer below the dashboard with mobile safe-area spacing
• contain Sessions tray overflow and hide the chart selector's native scrollbar
• place Sessions help and close controls in the upper-right on narrow screens
• add component regression coverage and document responsive behavior

Why

Intrinsic content widths were expanding the dashboard and Sessions detail beyond narrow viewports, while the fixed footer overlaid the final controls. The Sessions chart selector also exposed a native horizontal scrollbar.

Validation

• visually audited dashboard widths at 320 px, 390 px, and 576 px
• bun run ci (270 tests, TypeScript, Biome with zero warnings, Tailwind diagnostics, production build)