feat: Implement in-app token swap modal for purchasing voting power

[bonustrack]

Objective

Implement a feature that allows users to buy voting power directly within the Snapshot UI by swapping tokens using CoW Swap protocol via the @cowprotocol/cow-sdk library.

Requirements

Modify the existing "Buy" button in the voting power modal, which currently redirects to an external site. Instead, it should open a new modal within our site containing a token swap form. You can see the voting power modal if you go here: https://snapshot.box/#/s:uniswapgovernance.eth/proposals log in and click on the button with a ⚡ icon.

The token swap form should have two inputs:

Sell input:

• Numeric field for the amount to sell.
• Includes a token selector dropdown, defaulting to ETH.

Buy input:

• Numeric field showing the amount to receive.
• The token is pre-selected from the voting power modal (non-editable).

Features

• Updating either the "Sell" or "Buy" amount automatically adjusts the other field based on current exchange rates using the @cowprotocol/cow-sdk.
• Display the user's current balances for both tokens.
• Include a "Max" button to set the "Sell" amount to the user's maximum available balance.
• Ensure that swap calculations and rates match those on CoW Swap.
• Users should be able to successfully complete the token swap on-chain to purchase voting power.

Recommendations

• Use the Sepolia testnet network for integration and testing to not pay any gas fee.
• Design the modal and form to align with the site's existing style and UI/UX patterns.
• Reuse existing components where appropriate (e.g., token selection modal). You can find an example of the token selection modal here: https://snapshot.box/#/s:fabien.eth/treasury/1/tokens when you click on the send button in top right of the page.
• Reference Uniswap's Swap Interface for user experience inspiration.

Nice to have

• Display dollar value equivalents for "Sell" and "Buy" amounts.
• Show exchange rate and slippage (price impact) information.
• Lazy-load the @cowprotocol/cow-sdk dependency for performance optimization.
• Offload heavy logic to a serverless function (e.g., Fleek or Cloudflare Workers).