Switch Contact Sales to Book a Demo in navbar

Summary

This PR updates the navbar "Contact Sales" button to "Book a Demo" and changes its destination from the pricing page to a Cal.com demo booking form. Additionally, it updates the demo form email field to explicitly indicate "Business Email" to align with the existing business email validation logic.

Changes:

• Navbar button text: "Contact Sales" → "Book a Demo"
• Navbar button URL: /pricing → REFLEX_DEV_WEB_LANDING_FORM_URL_GET_DEMO (Cal.com form)
• Demo form email field: "Email" → "Business Email"
• Email placeholder: "john@example.com" → "john@company.com"

Review & Testing Checklist for Human

• Test that clicking "Book a Demo" in the navbar redirects to the correct Cal.com demo booking form
• Verify the Cal.com URL REFLEX_DEV_WEB_LANDING_FORM_URL_GET_DEMO is the intended demo booking form
• Check that no other "Contact Sales" buttons were missed in the codebase
• Confirm the demo form email field shows "Business Email" on the pricing page (requires reflex-ui changes to be deployed)

Recommended test plan: Navigate to the site, click "Book a Demo" in navbar, verify it opens the correct Cal.com form, then check the pricing page demo form shows "Business Email" field.

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    NavbarPy["pcweb/components/docpage/<br/>navbar/navbar.py"]:::major-edit
    ConstantsPy["pcweb/constants.py"]:::context
    DemoFormPy["reflex_ui/blocks/<br/>demo_form.py"]:::major-edit
    PricingPage["pcweb/pages/pricing/<br/>header.py"]:::context
    CalCom["Cal.com Demo Form"]:::context
    
    NavbarPy -->|"imports demo URL"| ConstantsPy
    NavbarPy -->|"links to"| CalCom
    DemoFormPy -->|"used by"| PricingPage
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit
        L3[Context/No Edit]:::context
    end

classDef major-edit fill:#90EE90
classDef minor-edit fill:#87CEEB  
classDef context fill:#FFFFFF

Notes

• The demo form already has business email validation logic that rejects personal domains (gmail, outlook, etc.), so this UI change aligns the label with existing functionality
• Screenshots from testing show the navbar change works correctly and redirects to Cal.com
• The reflex-ui changes are in a separate repository and will need to be deployed/installed for the email field changes to appear in reflex-web

Link to Devin run: https://app.devin.ai/sessions/88f0336bfe0047c28356fdfa6b902f30
Requested by: Alek (@Alek99)