feat: inline Turbo Stream notifications for bulk and single-job actions

[eclectic-coding]

Closes #38

Summary

• Wraps the flash area in <div id="sqw-flash-container"> so Turbo Stream responses can target it
• Adds shared/_flash.html.erb partial for consistent flash markup
• Both Turbo Stream discard responses (jobs/destroy, failed_jobs/destroy) now prepend a "Job discarded." notice inline without a page reload
• All bulk action redirects that were missing flash messages now report the affected count: "3 jobs discarded", "2 jobs retried", etc.
• Single failed-job retry now surfaces a "Job retried." notice on redirect

Test plan

• Discard a single job via the row button — confirm "Job discarded." appears inline and auto-dismisses after 7 seconds
• Discard a single failed job — same check
• Use "Discard All" on jobs — confirm notice shows count on the redirected page
• Select and discard/retry multiple failed jobs — confirm count in notice
• Select and discard multiple jobs — confirm count in notice
• Retry a single failed job — confirm "Job retried." notice
• bundle exec rake passes clean

🤖 Generated with Claude Code