fix: improve withdrawal queue fairness

[Mosas2000]

Summary

Fixes withdrawal queue starvation under sustained high pool utilization by adding fair partial fulfillment, cancellable queued requests, and investor-facing wait estimates.

What Changed

• Added a dedicated withdrawal queue counter so request IDs no longer overwrite queue storage.
• Added cancellable queued withdrawals by token through cancel_withdrawal_request.
• Added estimate_withdrawal_wait to expose:
  • queue position
  • capital ahead
  • nearest active invoice due date
• Added max_withdrawal_queue_age_days to pool config with an admin setter.
• Updated withdrawal queue processing to distribute available liquidity pro-rata instead of strict FIFO.
• Preserved aged withdrawal requests as the priority group when processing repayments.
• Updated the portfolio page to show queued withdrawal position, capital ahead, and nearest invoice due date.
• Added tests for queue persistence, cancellation, wait estimates, and pro-rata partial fulfillment.

Why

The previous queue could leave late investors waiting indefinitely when utilization stayed near 100% and repayments were continuously redeployed. This change gives queued investors better visibility and ensures repayment liquidity is shared fairly instead of only serving the front of the queue.

Testing

• git diff --check passed.
• cargo test -p pool was not run because cargo is unavailable in the local environment.
• npm run typecheck was not run because tsc is unavailable in the local environment.

Fixes

Closes #574

[sanmipaul]

LGTM. Thanks for tackling this, the counter key fix, the checked-arithmetic helpers, and the aged-request priority concept are all the right calls. Please, fix CI/CD errors.