align accounts page with figma

[shreeyaadhikari]

ℹ️ Issue

Closes #296

📝 Description

Updated the Accounts page so it uses the same page shell as the rest of BRANCH (left navigation + top header + consistent content spacing).

Changes made:

1. Added the shared app shell to Accounts by rendering the same sidebar navigation and top header used on other pages.
2. Updated the Accounts page container/layout spacing to match existing site patterns.
3. Kept existing Accounts content behavior intact (staff sections and Add User modal), while only changing page structure/styling integration.

✔️ Verification

[github-actions]

🌿 ⏳ Creating preview environment… (logs)

[github-actions]

🌿 Preview environment — ready ✅

Open: https://d3nmtjoh6ir9ym.cloudfront.net/pr-331/
API: https://s6icgey5o3.execute-api.us-east-2.amazonaws.com/prod

Shared RDS + Cognito (prod data); DB migrations are not applied here — if this PR adds a migration, endpoints using the new columns will fail until it merges. New commits update this environment in place — a note is posted here on each update. Remove the test-environment label or close the PR to tear it down.