Improve Instances tab visibility

[svuppala2006]

Summary

Improves visibility of the Instances tab to help users discover and manage their deployed instances, preventing instance accumulation on the cluster.

Fixes #20

Changes

1. Instance Count Badge on Tab

• Shows instance count on the "Instances" tab (e.g., "Instances (5)")
• Badge updates in real-time as instances are created/deleted
• Uses existing badge styling patterns for consistency

2. Auto-Switch After Successful Deploy

• Automatically switches to the Instances tab when a deployment completes
• User immediately sees their newly created instance
• Improves deployment workflow UX

3. Persistent Indicator on Deploy Tab

• Shows "X instances running" banner on the Deploy tab
• Clickable link to jump directly to Instances tab
• Visible reminder that instances exist and may need cleanup

Test Plan

All tests passing:

• ✓ Build successful (no TypeScript errors)
• ✓ All 471 tests passing
• ✓ No ESLint errors

Manual Testing

1. Instance count badge:

   • Deploy an agent → verify "Instances (1)" badge appears
   • Deploy another → verify badge updates to "(2)"
   • Delete instance → verify badge decrements

2. Auto-switch after deploy:

   • Start deployment from Deploy tab
   • Wait for completion → verify automatic switch to Instances tab
   • Verify new instance appears in list

3. Persistent indicator:

   • With instances running, switch to Deploy tab
   • Verify "X instances running" banner appears
   • Click "View instances →" → verify switch to Instances tab

Implementation Notes

• Follows existing patterns: badge styling, state lifting, callback props
• No breaking changes
• Minimal performance impact (reuses existing 5-second polling)
• Future enhancements (bulk delete, stale detection) deferred per issue discussion