[#432] Zap v2 Frontend UI: Multi-token selector on TradingWidget

[realproject7]

Parent: #432

Depends on: #434 (lib/zap.ts must be updated first)

Summary

Update TradingWidget.tsx to support multiple input tokens on the buy tab.

Token Selector

Add token selector on buy tab with options:

• ETH — payable via Zap, no approval needed
• USDC — approve to Zap → mint (two-step)
• HUNT — approve to Zap → mint (two-step)
• PLOT — direct MCV2_Bond.mint (existing behavior, no Zap)

Default: ETH

UI

• Minimal toggle/dropdown matching existing design (monospace, text-xs, border tokens)
• Show balance for selected input token
• Show estimate in selected input token
• For ERC-20 inputs: "Approving..." → "Confirm in wallet..." flow (same pattern as current PLOT approve → mint)

Behavior

• ETH: writeContractAsync({ value: ethCost }) — payable, no approval step
• USDC/HUNT: check allowance → approve if needed → writeContractAsync() with no value
• PLOT: existing MCV2_Bond.mint flow unchanged (no Zap contract involved)
• Sell tab: unchanged — always returns PLOT via MCV2_Bond.burn
• isZapAvailable check remains (hides selector when Zap address is zero)
• Trade indexing (/api/index/trade POST) fires for all modes

Acceptance Criteria

• Token selector renders on buy tab (ETH, USDC, HUNT, PLOT)
• Balance displays correctly for each token
• Estimates display in selected input token
• ETH mode: single tx, no approval
• USDC/HUNT mode: approve → zap flow
• PLOT mode: existing behavior preserved
• Sell tab unchanged
• Build passes

Branch

Same branch as #434: task/432-zap-v2-frontend in plotlink repo (combine in one PR)

[realproject7]

Multi-token selector (ETH/USDC/HUNT/PLOT) on TradingWidget. PR #436 merged.