feat: responsive layout — tables scroll, grids collapse, headers stack on narrow viewports

[eclectic-coding]

Closes #39

Summary

• Adds sqw-page-header--split base CSS (was used in views but never defined — previously just stacked block elements)
• Adds _10_responsive.css with three breakpoints:
  • 768px — cache size grid and cache timeline (2-col) collapse to single column; stacked timeline charts gain a top border instead of left border
  • 640px — main padding reduced to 1rem; split page headers stack vertically; tables become display: block; overflow-x: auto for horizontal scroll; stats grid shrinks minmax from 160px → 120px; gem grid collapses to 1 column; truncate max-width reduced; period filter de-floats
  • 576px — header and subnav inner padding reduced; subnav wraps to multiple rows when needed

Test plan

• Resize browser to ~640px wide — confirm tables scroll horizontally, page headers stack, padding reduces
• Resize to ~768px — confirm cache size grid and timeline collapse to single column
• Resize to ~480px — confirm header padding tightens and subnav wraps cleanly
• Check all three sections (Queue, Cache, Cable) at narrow widths
• Confirm no visual regressions at full desktop width
• bundle exec rake passes clean

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