fix(notebook): render db selector dropdown via portal so it isn't clipped

[ymadd]

Summary

• The notebook SQL cell's database selector dropdown (introduced in feat: Make database selector dropdowns scrollable after 10 entries #160) is rendered with position: absolute inside NotebookCellWrapper, which has overflow-hidden. As a result, when a connection manages many databases (e.g., a single MySQL host with dozens of schemas), the dropdown's max-h-[230px] scroll area is clipped by the cell's bounding rect — particularly noticeable on short cells (no result yet, collapsed neighbors, or the last cell in a notebook), where lower entries become unreachable and the scrollbar isn't visible.
• This PR renders the dropdown through a React portal (createPortal to document.body) with position: fixed, computed from the trigger button's getBoundingClientRect(). Position is also recomputed on scroll (capture) and resize, matching the existing pattern in src/components/ui/Select.tsx.
• Visual styling, max-h-[230px] overflow-y-auto behavior, backdrop click-to-close, and the show-only-when-selectedDatabases.length > 1 condition are all preserved.

Why

Without this change, overflow-hidden on the cell wrapper visually clips the absolutely-positioned dropdown regardless of its inner overflow-y-auto, defeating the scroll added in #160 whenever the cell is shorter than the dropdown.

Test plan

• pnpm typecheck passes
• pnpm tauri dev — connect to a MySQL host with many databases (>12) and select multiple in the connection dialog
• Open a Notebook tab, add an SQL cell, click the database selector in the cell header
• Confirm the dropdown extends past the cell's visual bottom and scrolls to reach all databases
• Confirm position updates correctly when the notebook scroll container is scrolled and on window resize
• Confirm clicking outside / on the backdrop closes the dropdown

[debba]

Good catch, will merge it, thanks for your contribution.