Add stale-while-revalidate caching to the investor portfolio query for instant repeat visits

[Chucks1093]

Summary

The investor portfolio query refetches on every visit, showing a spinner even when the data has not changed. Setting a staleTime should serve the cached portfolio instantly and revalidate in the background so the page feels responsive on repeat visits.

Scope

• Set staleTime to 60 seconds on the investor portfolio query
• Serve cached positions immediately on repeat visits without a spinner
• Show a subtle 'Refreshing…' indicator only when a background refetch is in progress
• Invalidate the cache immediately after a new investment is confirmed
• Confirm stale data is not served if the user has been away for more than 60 seconds

Acceptance Criteria

• Cached portfolio shown instantly on repeat visits within 60 seconds
• Background refetch indicator visible during revalidation
• Cache invalidated after a confirmed investment
• Spinner not shown when serving a fresh cached response

ETA: 12 hours

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Coordinate on Telegram

[Stephan-Thomas]

@Stephan-Thomas has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

hi, i would like to work on this.

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Congratulations, @Stephan-Thomas! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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