feat(earn): add stake/unstake dialog and mutation

[precious-akpan]

Closes #310

Summary

Added the StakeDialog component and its corresponding mutation hook for users to manage their SO4 stake.

Details

• Implemented StakeDialog supporting toggling between Stake and Unstake actions.
• Connected the dialog to the newly created useStakeMutation hook which calls stakeSO4 and unstakeSO4 directly.
• Added duration multiplier selection buttons (1x, 1.5x, 2x) for staking.
• Implemented client-side input validation rejecting negative or invalid amounts.
• Wrote extensive tests covering dialog tab switches, input handling, multiplier changes, and submission states.

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