fix(dashboards): make timezone list scrollable when not searching

[rohithreddykota]

• The timezone selector only capped and scrolled the search-results group (max-h-72 overflow-y-auto); the pinned/available-zones group rendered on open was uncapped, so projects with many configured timezones overflowed off-screen.
• Applied the same max-h-72 overflow-y-auto constraint to the pinned group in both Zone.svelte and ZoneContent.svelte (the latter used by RangePickerV2), so the default list scrolls just like the search results.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code