docs: update demo screenshot to the new UI

[MohamedAklamaash]

Replaces demo/graph.png (referenced in the README) with a screenshot of the new CodeGraph UI — the redesigned app shell, indigo design system, user-defined graph view, and grounded AI chat answering "What happens after login?".

Follow-up to #1 (which is already merged); this carries only the image change.

🤖 Generated with Claude Code

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