fix: add amount input and validation to MilestoneFundButton (#83)

[rafaio1]

Fixes #83.

• Add an amount input field to MilestoneFundButton, mirroring the existing pattern in PoolDepositButton within the same file.
• Validate the entered amount before submission: rejects empty, non-finite, zero, negative, and over-precision (>7 decimals) values client-side.
• Include the validated amount in the POST /milestones/:id/fund request body alongside funderAddress. Previously, no amount was sent at all, making partial funding impossible from the UI despite the progress bar modeling incremental contributions.
• Backend contract note: The fix assumes the backend endpoint accepts or requires an amount field (consistent with the maintenance pool deposit endpoint). If the current backend silently interprets a missing amount as 'fund entire remaining budget,' this change makes that behavior explicit and user-controlled rather than implicit.
• TypeScript compiles cleanly with npx tsc --noEmit.

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