Add configurable collections page size with localStorage persistence (#297)

[srimon12]

Summary

Fixes #297
Fixes #290

Adds a configurable page size dropdown to the Collections page, addressing #297 and #290.

Changes

• Replaced the hardcoded PAGE_SIZE = 5 with a pageSize state variable that initializes from localStorage
• Added a Select dropdown (FormControl + MenuItem) in the Collections header with options: 5, 10, 25, 50
• Page size persists to localStorage under qdrant-web-ui-collections-page-size
• Changing page size resets to page 1 and re-fetches the current page
• Pagination controls dynamically recalculate based on the selected page size

Files Changed

• src/pages/Collections.jsx — 40 insertions, 7 deletions

Verification

• 123/123 tests pass, no lint errors
• Manually tested in dev server: dropdown renders, switching page size updates the displayed collections and pagination correctly, localStorage persistence confirmed