feat: timezone-corrected timestamps

[eclectic-coding]

Summary

• Adds a timestamp_controller.js Stimulus controller that reads <time datetime="ISO8601"> elements and reformats them in the browser's local timezone using Intl.DateTimeFormat
• Adds a local_time view helper that renders <time> elements with UTC ISO 8601 datetime attributes and UTC fallback text (degrades gracefully without JS)
• Updates all 18 timestamp displays across jobs, failed jobs, history, processes, queues, recurring tasks, cache entries, cable channels, and the dashboard to use the new helper
• Relative timestamps (oldest cache entry, oldest cable message, cable message list) use Intl.RelativeTimeFormat with the full local timestamp exposed on hover via title

Test plan

• All 256 existing request specs pass (bundle exec rake)
• Jobs index shows enqueued-at timestamps in local timezone
• Jobs show page displays enqueued/scheduled/finished times in local timezone
• Failed jobs index and detail page show local timestamps
• History page shows finished-at in local timezone
• Processes page shows last heartbeat in local timezone
• Recurring tasks show next run / last run in local timezone
• Cache entries index and detail show created-at in local timezone
• Cable channel browser shows last message time in local timezone
• Cable message list shows relative time with full local timestamp on hover
• Dashboard oldest cache / oldest cable message shows relative time with hover title
• Fallback (UTC text) visible when inspecting the raw HTML <time> element

Closes #58

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