docs: add release notes for 0.2.0 and 0.2.1

Add RELEASES.md covering changes from 0.1.x to 0.2.1:
server helpers, tool visibility, theming, display modes,
peer dependencies, Windows compat, SSE transport, E2E tests

Author: claude

RELEASES.md

@@ -0,0 +1,37 @@
+# Release Notes
+
+## 0.2.1
+
+Changes from 0.1.x to 0.2.1.
+
+### Highlights
+
+- **Server helpers** — New `registerAppTool()` and `registerAppResource()` simplify server setup with proper type safety. `connect()` now defaults to `PostMessageTransport(window.parent)`, enabling simpler initialization with just `await app.connect()` by @ochafik in [#165](https://github.com/modelcontextprotocol/ext-apps/pull/165)
+- **Tool visibility control** — New `visibility` array field controls whether tools are visible to the agent, apps, or both. Restructured `_meta` format from flat `"ui/resourceUri"` to nested `_meta.ui.resourceUri` by @jonathanhefner in [#131](https://github.com/modelcontextprotocol/ext-apps/pull/131)
+- **Host-provided theming** — Apps receive 36 standardized CSS variables for colors, typography, and spacing via `styles.variables`, enabling visual consistency with the host by @martinalong in [#127](https://github.com/modelcontextprotocol/ext-apps/pull/127)
+- **Display mode requests** — Apps can request display mode changes (e.g., fullscreen) via `requestDisplayMode`, with hosts able to accept or reject by @martinalong in [#152](https://github.com/modelcontextprotocol/ext-apps/pull/152)
+- **Custom fonts support** — Apps can receive custom fonts via `styles.css.fonts` and apply them using `applyHostFonts()` or `useHostFonts()` helpers by @martinalong in [#159](https://github.com/modelcontextprotocol/ext-apps/pull/159)
+
+### API Changes
+
+- **MCP SDK as peer dependency** — Consumers control their SDK version, reducing duplication by @ochafik in [#168](https://github.com/modelcontextprotocol/ext-apps/pull/168)
+- **React as peer dependency** — Supports React 17, 18, and 19 by @ochafik in [#164](https://github.com/modelcontextprotocol/ext-apps/pull/164)
+- **Renamed request methods** — Removed `send` prefix: `openLink()` and `teardownResource()`. Deprecated aliases maintained by @ochafik in [#161](https://github.com/modelcontextprotocol/ext-apps/pull/161)
+- **Optional Client in AppBridge** — Enables custom forwarding scenarios without direct MCP client access by @ochafik in [#146](https://github.com/modelcontextprotocol/ext-apps/pull/146)
+- **Zod v3 and v4 support** — Bring your own Zod by @alpic-ai in [#49](https://github.com/modelcontextprotocol/ext-apps/pull/49)
+
+### Platform & DX
+
+- **Windows compatibility** — Bun as optional dependency with automatic setup, `cross-env` for examples. Just run `npm install` by @ochafik in [#145](https://github.com/modelcontextprotocol/ext-apps/pull/145)
+- **Widened `@oven/bun-*` version range** — Lowered the minimum from `^1.3.4` to `^1.2.21`, helping contributors whose registries may not have the latest packages by @ochafik in [#176](https://github.com/modelcontextprotocol/ext-apps/pull/176)
+- **SSE transport support** — All examples now support both stdio and HTTP transports with SSE endpoints for older clients by @ochafik in [#136](https://github.com/modelcontextprotocol/ext-apps/pull/136)
+- **Playwright E2E tests** — Screenshot golden testing across all 9 example servers with parallel execution (~28s) by @ochafik in [#115](https://github.com/modelcontextprotocol/ext-apps/pull/115)
+- **hostContext exposure** — Access `hostContext` directly from the `App` class by @ochafik in [#139](https://github.com/modelcontextprotocol/ext-apps/pull/139)
+
+### Bug Fixes
+
+- **Responsive UIs** — Fixed narrow viewport handling for mobile and sidebar experiences by @ochafik in [#135](https://github.com/modelcontextprotocol/ext-apps/pull/135)
+- **Sandbox notification fix** — Fixed `sandbox-ready` notification name to match implementation by @ochafik in [#160](https://github.com/modelcontextprotocol/ext-apps/pull/160)
+- **Fixed non-UI tool registration** — Use `server.registerTool` for tools without UI and fix missing imports across examples by @ochafik in [#173](https://github.com/modelcontextprotocol/ext-apps/pull/173)
+
+**Full Changelog**: https://github.com/modelcontextprotocol/ext-apps/compare/v0.1.1...v0.2.1