fix(diem-staking): approve exact stake amount instead of unlimited DIEM allowance

[Augustas11]

Summary

• Replaces maxUint256 in useApproveDiem with the precise amount the user is staking (parseEther(amountDiem))
• Updates the StakeCard.tsx call-site to pass props.amt through to the hook
• Removes the now-unused maxUint256 import from viem

Why

Unlimited ERC-20 allowances are a known blast-radius amplifier. If the staking proxy is ever upgraded with a bug or a key is compromised, a standing maxUint256 approval gives an attacker full access to every user's DIEM balance. Approving exactly the stake amount eliminates that risk. Users will now see "Approve X DIEM" in their wallet instead of "Approve unlimited".

Test plan

• Typecheck passes (pnpm --filter diem-staking exec tsc --noEmit) — verified clean locally
• Approve button flow: enter an amount, click Approve DIEM, confirm wallet shows the specific amount (not unlimited), allowance refetches, Stake button becomes active
• Stake-without-prior-allowance regression: fresh wallet session shows approve step correctly
• No test suite exists for apps/diem-staking yet — a Wagmi-mock unit test asserting approve receives parseEther(amount) rather than maxUint256 is a recommended follow-up

Fixes BUG-155.

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