feat(dao): accept vote spend multiples

[dnlbui]

What Changed

This PR lets voters enter DAO vote spend as a positive whole-number multiple of the proposal's minimum spend instead of entering a raw LIB amount.

• ProposalInfoModal converts the selected multiple into the exact LIB spend using the proposal minimum and current network pricing.
• Option weights remain independent and continue to define the proportional allocation across proposal choices.
• The preview shows the calculated spend, timing multiplier, total estimated voting power, and per-option allocation.
• The spend multiple defaults to 1 and is revalidated before the vote transaction is signed.

Fixed Flow

1. The voter assigns weights to one or more proposal options.
2. The voter chooses a whole-number minimum-spend multiple.
3. The client calculates and previews the resulting LIB spend and voting power.
4. After confirmation, the client revalidates the inputs and submits the calculated spend with the independent option weights.

Why

Entering a raw LIB amount made voters calculate the proposal minimum themselves. Expressing spend as a minimum-spend multiple makes valid amounts easier to choose while preserving the server's existing spend and weights transaction fields.

Validation

• git diff --check 64a05aee0d66b0afb022676bce4297ac87e1656a
• node --check app.js
• node --check dao.js
• Client E2E GitHub check

Follow-up: #1568

Closes #1515