feat: contextual empty states with actionable hints across all list views

[eclectic-coding]

Closes #37

Summary

• Adds sqw-empty__title and sqw-empty__hint CSS sub-elements for structured empty state content
• All 11 empty states across Queue, Cache, and Cable sections updated with a bold title and a muted contextual hint
• Search/filter empty states include a "Clear search" or "Clear filters" link
• Section-specific context: processes → "Start a Solid Queue worker"; failed jobs → "All clear"; stats/history → link to active jobs; cable messages → link back to channels
• Jobs empty partial is status-aware (ready / scheduled / claimed / blocked)

Test plan

• Visit Jobs with each status tab on an empty database — confirm status-appropriate hint
• Search for a non-existent job — confirm "Clear search" link appears
• Visit Failed Jobs with no failures — confirm "All clear" message
• Visit Processes with no workers — confirm worker hint
• Visit History with filters active and no results — confirm "Clear filters" link
• Visit Cable with no messages — confirm hint text
• bundle exec rake passes clean

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