Bundled Postman-like HTTP client extension

[JDonaghy]

Tracking issue. Not a current priority. Depends on #146 — plugin API extension.

What

A bundled Lua extension for vimcode that provides HTTP request building and response viewing — equivalent to VSCode's "REST Client" or "Thunder Client" extensions. Lives in `src/core/extensions/` alongside the other bundled extensions.

Features

MVP:

• Collections/folders tree (`TreeView`) — reusable request definitions
• Request builder (`Form`) — URL, method dropdown, headers table, query params table, body editor (`TextEditor` with JSON/XML syntax)
• Send button + async HTTP
• Response viewer — pretty-printed JSON/XML (`TextDisplay`), headers list, status + timing + size
• Request history (`ListView`)
• Basic environment support (variable substitution in URL/headers/body)

Stretch:

• Environments switcher (`Palette`)
• Pre/post-request Lua hooks for auth signatures, assertions, token refresh
• Import from Postman collection JSON
• Export as cURL
• WebSocket support
• gRPC support (pipe-dream)

Why this, why now, why not now

• Why this: Validates that plugin-authored rich-UI extensions work on quadraui. Proves vimcode can host a real productivity app as a plugin (not just annotations).
• Why not now: Depends on #TBD (plugin API surface). Building this before that API stabilizes would mean throwing it away.
• Why track it: So the plugin API design is informed by a concrete consumer, even if we don't build it for a year.

Non-goals

• No JS scripting (unlike Postman). Pre/post-request hooks use Lua via vimcode's existing mlua runtime.
• No cloud sync / team workspaces (local files only; users can check collections into git)
• No visualizers beyond what quadraui `TextDisplay` supports (pretty JSON / XML / images). No custom HTML renderers.

Dependencies

1. Extend Lua plugin API to expose quadraui primitives #146 — Plugin API extension (blocking)
2. `vimcode.http` async HTTP API in vimcode (listed under #TBD but logically a separate piece of work)
3. quadraui primitives available: `Form`, `TreeView`, `TextEditor`, `TextDisplay`, `ListView`, `Palette`, `Dialog`

Priority

Low. Re-evaluate when:

• quadraui 1.0 is published
• Plugin API extension (#TBD) is done
• There's user demand or competing extensions for vimcode

References

• VSCode REST Client: https://marketplace.visualstudio.com/items?itemName=humao.rest-client
• VSCode Thunder Client: https://marketplace.visualstudio.com/items?itemName=rangav.vscode-thunder-client
• `docs/UI_CRATE_DESIGN.md` — Postman covered in \§5 as a validation case