Add interactive demo interface section with styles and auto-rotating screens

[helloroman-bez]

Motivation

• Provide an embedded visual demo of the product UI to showcase dashboard, leads list, and chat flows for visitors.
• Improve conversion by giving a realistic preview of monitored platforms and lead scoring directly on the landing page.
• Offer an accessible, responsive visual component that complements existing hero and "how it works" blocks.

Description

• Added a new section with id="demo" containing a platform strip, tabbed demo shell, three demo screens (Dashboard, Leads list, Chat), and dot indicators.
• Introduced CSS rules for the demo block and responsive adjustments, including .demo-shell, .demo-tab, .demo-screen, .demo-metric, and related classes to style the dark demo UI and grid layout.
• Included an inline SVG chart and static demo cards for leads and chat content to represent UI elements and metrics.
• Added client-side behavior with setDemoScreen, startDemoRotation, and event listeners to enable auto-rotation of screens every 5 seconds and manual tab/dot switching.

Testing

• No automated tests were added or executed for this change.

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Codex Task