feat: add subscription management panel (#163)

[gTanusri]

Summary

• Adds a SuperAdmin-only subscription management panel.
• Supports creating, editing, deleting, and searching subscription plans.
• Supports creating, editing, deleting, and searching subscription features.
• Supports assigning features to plans and assigning plans to restaurants.
• Adds current restaurant subscription lookup.
• Adds SuperAdmin-only sidebar visibility for subscription management.

Issue

Closes #163

Verification

• npm run build
• npm run lint