Add helper for displaying total portfolio value across all held keys

[Chucks1093]

Summary
Users who hold keys in multiple creators have no way to see their combined portfolio value at a glance. A helper that sums the current value of all held key positions and displays a total gives holders an overview without manual calculation.

Scope

• add a helper that aggregates the current value of each held key position
• display the total portfolio value on the holdings page
• handle missing or stale price data gracefully with a loading or unavailable state

Acceptance Criteria

• the total portfolio value is displayed and updates when prices refresh
• missing price data shows a loading or unavailable indicator rather than a wrong total
• the helper handles zero holdings as a zero total without error

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[probablyABug]

@probablyABug has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

hi, i would like to work on this.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @probablyABug to this issue.

[Kappa16]

@Kappa16 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would like to work on this, expect PR in few hours stat.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Kappa16 to this issue.