fix(charts): render consistent session chart dividers

[ridecontrol]

Changes

• Replace fractional-width SVG chart boundaries with single CSS borders.
• Draw each session chart separator once, keeping the heart-rate divider consistent with the other metrics.
• Preserve dashed grid guides and theme-specific colors.

Verification

• Visually checked stacked charts in dark and light themes, desktop and mobile sizes, and fractional CSS zoom.
• bun run ci passed for commit 76ff812: 404 tests, Biome, Tailwind diagnostics, TypeScript, and production build.