fix: Signup modal visual and interaction bugs after React migration

[inureyes]

Description

After PR #5356 migrated the Signup modal from Lit-Element to React, three visual/functional issues were identified.

Problems

1. Incorrect Font Size

• Form labels display with different font sizes compared to the previous Lit version
• Shadow DOM CSS property inheritance may cause Lit parent styles to leak into the React component
• Fix: Set explicit fontSize on BAIModal body to prevent inheritance issues

2. No Gap Between Footer Buttons

• Cancel and Signup buttons in the modal footer are touching each other with no spacing
• Caused by BAIFlex having a default gap of 0
• Fix: Add gap="sm" to BAIFlex in the footer

3. React Modal Click Events Not Working

• BAIModal (Ant Design Modal) renders its portal to document.body by default, placing it outside the Shadow DOM
• StyleProvider container={shadowRoot} injects CSS only inside the Shadow DOM, so styles don't apply to the portal
• z-index conflict between Lit's backend-ai-dialog (z-index: 1001) and BAIModal (z-index: 1001)
• Fix: Add getContainer={false} to all modals to render inline within the Shadow DOM

Files to Modify

• react/src/components/SignupModal.tsx
• react/src/components/TermsOfServiceModal.tsx
• react/src/components/PrivacyPolicyModal.tsx

Related PR

• feat(FR-991): migrate signup modal from Lit-Element to React #5356