fix(frontend): missing accessory bar styles for 430-768px tablet viewport

[maupet]

Summary

• The keyboard accessory bar buttons (settings gear, commands, Terminal/Transcript toggle, project picker, hamburger) had dark styling only in the <430px (phone) and 768-1023px (large tablet) breakpoints
• The 430-768px tablet range had no styles at all, causing buttons to render with default white browser styling — ugly and hard to tap
• Moves shared accessory bar component styles to the global scope in mobile.css so they apply at all sizes where the file loads (<1024px). Breakpoint-specific blocks still override sizes as needed.

Before / After

Before (430-767px)	After (should match <430px)
White unstyled buttons, hard to tap	Dark themed buttons matching phone layout

Test plan

• Open web UI at 430-767px viewport width
• Verify accessory bar buttons have dark backgrounds with subtle borders
• Verify Terminal/Transcript segmented toggle is dark-themed, icons only
• Verify commands drawer (Search commands...) has dark background
• Verify no regressions at <430px (phone) and 768-1023px (large tablet)
• Verify no regressions at >1024px (desktop)

🤖 Generated with Claude Code

[maupet]

this bugged me for so long. Until I discovered I could swipe across sessions, but still it's useful (and nicer) to have it properly styled on mobile