UX: Warn or block when creating a policy with no parameter changes

[MaxGhenis]

Problem

Users can currently create a policy with policy_json: {} - effectively identical to current law. This happens when they go through the policy creation flow without adding any parameter changes.

Proposed Solution

On the policy review/submit screen, if parameters is empty:

Option A: Block (Recommended)

• Disable the "Create policy" button
• Show message: "Add at least one parameter change to create a policy"

Option B: Warn

• Show warning when clicking "Create policy": "This policy has no changes from current law. Are you sure?"
• Allow user to proceed if intentional

Why This Matters

This catches users who:

1. Forgot to click "Add parameter" after modifying values (see UX: Make policy parameter changes more intuitive - remove need to click 'Add parameter' #604)
2. Accidentally removed all their changes
3. Are confused about the flow

Related: #604 (make parameter addition more intuitive)