feat(nav): alerts bell icon + active-alert badge (#582)

[vivekchand]

Closes #582

Summary

• Adds a bell icon button to the nav bar (replacing the commented-out budget button); clicking it navigates to the Alerts tab
• Adds a red badge on the bell that shows the count of active (unacknowledged) alerts, hidden when count is 0
• Adds the same badge to the Alerts nav tab, matching the existing Approvals tab pattern
• Extends the existing checkActiveAlerts() poller in app.js (already runs every 30 s) to update both new badge elements — no new polling interval added

Test plan

• Start dashboard with no active alerts — bell icon visible, no badge shown, Alerts tab has no badge
• Trigger an alert (e.g. via the budget monitor or a custom rule) — badge appears on bell and Alerts tab with correct count
• Dismiss all alerts via the banner — badges disappear within 30 s
• Count > 99: badge shows "99+" instead of the raw number
• Clicking the bell navigates to the Alerts tab

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