fix: make bookmarks dropdown scrollable

[nishantmonu51]

The bookmarks dropdown in the dashboard header had no height bound, so a project with many managed bookmarks rendered a list that ran off the bottom of the viewport with no way to reach the lower entries (`DropdownMenuContent` sets no `max-height`/`overflow`, and bits-ui's `avoidCollisions` flips or shifts a floating menu but never shrinks it).

• Bound `DropdownMenuContent` in `Bookmarks.svelte` to `min(600px, --bits-floating-available-height)` so it fits the space below the trigger.
• Keep "Bookmark current view" and the search box pinned; only the Your / Managed bookmark lists scroll.
• Section labels are `sticky` so the heading stays visible while scrolling.

Scoped to the bookmarks menu rather than the shared `DropdownMenuContent` component to avoid changing every menu in the product.

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!