feat: Implement member filter sidebar (issue#34)

[IshimweIsaac]

Overview

Closes #34

This PR implements the Member Filter Sidebar on the /members directory page. The sidebar allows users to filter the member list by their tech stack and availability, with all filter states preserved in the URL search parameters to ensure shareability.

Changes Made

• Added components/members/filter-sidebar.tsx as a client component.
• Implemented FILTER BY STACK checkboxes utilizing the existing styled Checkbox UI component.
• Implemented AVAILABILITY dropdown using a native select element.
• Added a CLEAR ALL FILTERS outlined button.
• Synced component state with URL parameters (?stack=...&availability=...) on mount and when changes occur.

Acceptance Criteria

• Checkbox states sync with URL on mount + on change
• Selected checkbox uses cyan (Checkbox component already styled)
• Selected dropdown option is highlighted
• CLEAR ALL FILTERS removes all params from URL

Screenshots