feat: cable timeline — 24-hour message volume chart

[eclectic-coding]

Closes #35

Summary

• Adds CableTimeline model that buckets SolidCable::Message records into 24 hourly slots over the last 24 hours
• Renders a full-width bar chart on the Cable overview page using the existing build_sparkline_svg helper; each bar has a hover tooltip showing the exact message count for that hour
• Adds cable_messages_timeline_svg helper and sqw-timeline-chart--cable / sqw-timeline-grid--single CSS modifiers so the cable chart uses var(--info) and spans full width (vs. the 2-column cache layout)
• Adds margin-bottom to .sqw-timeline-grid to match existing margin-top

Test plan

• Start the dummy app (cd spec/dummy && bin/rails server) and visit /solid_stack/cable
• Confirm the 24-hour bar chart renders below the stats and above the search form
• Hover bars to verify tooltips show correct hour labels ("X messages this hour" / "X messages Nh ago")
• Confirm empty buckets render at reduced opacity
• Visit /solid_stack/cache and confirm the Cache timeline is unaffected
• bundle exec rake passes clean

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