Add maximum holding cap warning on buy panel when approaching wallet limit #961

Summary

Display a warning on the buy panel when the connected wallet's current holding is approaching or at the maximum holding cap for a creator key.

Scope

• Fetch wallet holding and max cap on key detail page load
• Progress indicator showing current holding as percentage of cap
• Warning banner when holding exceeds 80% of cap
• Buy input capped at remaining allowable quantity
• Cap reached state disables buy button with clear message

Acceptance Criteria

• Progress indicator shows correct percentage of cap used
• Warning banner appears at 80% of cap
• Buy quantity input automatically limited to remaining capacity
• Buy button disabled with cap reached message at 100%
• Cap display updates after each successful buy

ETA

12 hours