Inflow vault - withdrawal partial fills

### Current Behavior

Withdrawals from Inflow vaults are all-or-nothing:
- If sufficient funds available -> user receives full amount immediately
- If insufficient funds -> entire amount queued, user receives nothing

### Proposed Change

Implement partial withdrawals: immediately transfer available funds to user, then queue only the remaining amount.

**Benefit:** Improved UX by reducing wait times when liquidity is partially available.

**Note:** It will require parallel front-end work

Start with the Inflow vault withdrawal flow and its existing all-or-nothing queueing behavior; the issue names no files or tests, so trace the relevant backend entry point and coordinate the parallel front-end work. Done means available funds transfer immediately, only the remainder is queued, and both partial- and insufficient-liquidity paths are covered.

Written by the indexing model from the issue text.