Redesign UI with shadcn/ui (Tailwind + Radix) and add Go tests

[dropdevrahul]

Replace the MUI/Emotion frontend with a Tailwind CSS + shadcn/ui design system for a cleaner, dev-tool "instrument panel" aesthetic.

Frontend:

• Remove @mui/, @emotion/, react-json-pretty, @fontsource/inter
• Add Tailwind, Radix primitives, lucide-react, IBM Plex Sans/Mono
• New ui/ component kit (button, input, textarea, select, tabs, dialog, card, badge, separator, label) + cn helper and @/* alias
• Custom json-view to highlight JSON responses
• Move the add-header action to the section title
• Fix: headers now sent as a plain object (the old Map serialized to {})
• Fix: cURL import populates header rows and body; response follows the active tab; add send loading state; fix the <title> typo

Tests:

• Add Go unit tests for the curl parser (parse_curl_test.go) and the HTTP header helper (app_test.go); run with go test ./...

Docs:

• Add CLAUDE.md; keep AGENTS.md