Compact the mobile race-dialog header

[shaug]

TL;DR

Removes the obsolete mobile stacking rule so race titles wrap beside the existing icon actions without leaving a near-empty header band.

Summary

• preserve the compact mobile padding while allowing the base flex layout to keep actions top-right
• add browser-backed coverage at 320, 375, and 414 px using the longest race title
• verify the sticky header keeps its size and position while the dialog scrolls

Validation

• uv run pytest tests/test_rendering.py -k "phone_dialog_header or phone_dialog_metrics" -q
• make check
• repository-owned review-code-change: clean at e3b60a9

Fixes #174