chore: CSS audit — dark mode badge overrides and surface-hover variable

[eclectic-coding]

Closes #36

Summary

Audit findings:

• No dead CSS — all 110 defined classes are actively used in views, helpers, or JS (many are set via dynamic Ruby string interpolation which the naive grep misses)
• Badges not dark-mode aware — all 10 badge colour variants (ready, scheduled, claimed, blocked, failed, paused, queue, worker, supervisor, dispatcher) used hardcoded light-mode hex values with no dark overrides; in dark mode they were rendering as bright light-coloured chips on a dark background
• Table row hover hardcoded — #f9fafb showed as a near-white flash on hover in dark mode

Fixes applied:

• Added --surface-hover: #f9fafb to :root; overridden to #1c2128 in [data-theme="dark"]
• sqw-table tbody tr:hover now uses var(--surface-hover)
• Added 10 [data-theme="dark"] .sqw-badge--* overrides using rgba() tints of the semantic colour variables so badges read correctly in both themes

Test plan

• Toggle dark mode — confirm all badge variants (processes page, jobs page) render correctly with tinted dark backgrounds
• Hover table rows in dark mode — confirm subtle dark hover, not a white flash
• Confirm all badges still look correct in light mode
• bundle exec rake passes clean

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