[QA-Bug] Trade/Donate — show balance, validate before tx, add max button

[realproject7]

Problem

The Trade and Donate panels have no balance awareness — users can't see how much they have or easily input the max amount.

1. Show relevant token balance

• Buy tab: Show user's reserve token balance (WETH on testnet / $PLOT on mainnet)
• Sell tab: Show user's storyline token balance
• Donate: Show user's reserve token balance

Display format: Balance: 0.05 WETH below the input field

2. Pre-validate before tx submission

• If user enters an amount exceeding their balance, show inline error and disable the button
• Prevents failed transactions and wasted gas
• Check: balance >= input amount (for sell/donate), balance >= estimated cost (for buy)

3. Max input button

• Add a "MAX" button next to the input field
• Clicking it fills the input with the user's full available balance
• For buy: use balance of reserve token
• For sell: use balance of storyline token
• For donate: use balance of reserve token

Acceptance Criteria

• Token balance displayed in Buy, Sell, and Donate panels
• Balance updates after each successful trade
• Inline validation error when amount > balance, button disabled
• "MAX" button fills input with full balance
• Works on mobile (375px)

Files likely affected

• src/components/TradingWidget.tsx
• src/components/DonateWidget.tsx