feat(donate): add a Max button with a fee reserve

[Favourof]

Description

close #237.

The donation form previously lacked a "Max" button. Additionally, the insufficient balance validation did not account for the native XLM network fee or minimum base account reserve. If a donor tried to stream their entire XLM balance, the frontend check would pass but the transaction would fail on-chain during signing/submission.

This PR introduces a "Max" quick-fill button and reserves a safe balance buffer for native XLM transactions.

Changes

• In src/lib/stellar.ts:
  • Exported NATIVE_ACCOUNT_RESERVE = 10_000_000n (1.0 XLM).
  • Exported NATIVE_FEE_ESTIMATE = 1_000_000n (0.1 XLM).
  • Exported NATIVE_FEE_RESERVE = 11_000_000n (1.1 XLM buffer).
• In src/lib/format.ts:
  • Added and exported formatAmountInput(raw: bigint, decimals: number).
• In src/components/donate/CreateStreamForm.tsx:
  • Added a "Max" button next to the existing preset buttons.
  • For native XLM, clicking "Max" deducts NATIVE_FEE_RESERVE so the account retains necessary reserve and fee funds.
  • Updated the balance check so users attempting to deposit more than balance - NATIVE_FEE_RESERVE are warned about insufficient reserve buffer before signing.
  • For non-native/custom tokens, "Max" populates the entire token balance scaled by token decimals.
• Added test coverage in:
  • src/lib/format.test.ts: Added unit tests for formatAmountInput.
  • src/components/donate/CreateStreamForm.test.tsx: Added tests for native fee reserve deduction, non-native token max balance, and boundary conditions.

How to Test

1. Run npx vitest run src/components/donate/CreateStreamForm.test.tsx src/lib/format.test.ts.
2. Connect a wallet and verify clicking "Max" with XLM reserves 1.1 XLM.
3. Switch to a custom token and verify "Max" fills the full token balance.

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