Make the component installable as a git dependency

`component/package.json` describes the built component as a package
(`@element-hq/element-call-component`: entry, stylesheet subpath, types,
peer dependencies) so that a host can depend on
`github:element-hq/element-call#<ref>&path:/component`. Its `prepare`
script builds on install, since nothing is published yet.

For that the component build now lands in `component/dist` instead of the
repository's `dist`, and `pnpm build:component` also emits the type
declarations (`component/tsconfig.build.json`, `build:component:types`),
which previously had to be produced by hand.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -217,7 +217,8 @@ See also:
 
 Element Call can also be embedded directly into another React application
 rather than being loaded in an iframe as a widget. `pnpm build:component`
-builds it as a library, and
+builds it as a library into `component/dist` (the bundle, its stylesheet and
+type declarations), and
 
 ```sh
 pnpm dev:component
@@ -240,6 +241,20 @@ what is inside it, with `html`, `body` and `:root` standing for that root (see
 `component/build/scopeStylesToRoot.ts`). A host's own page keeps its styles,
 and Element Call brings its own fonts and design tokens along.
 
+The package is not published yet. A host installs it as a git dependency on the
+`component` directory of this repository,
+
+```json
+"@element-hq/element-call-component": "github:element-hq/element-call#main&path:/component"
+```
+
+whose `prepare` script runs the build on install (the host's pnpm has to allow
+that: `allowBuilds` in its `pnpm-workspace.yaml`). It imports the component from
+`@element-hq/element-call-component` and the stylesheet from
+`@element-hq/element-call-component/style.css`, and has to provide `react`,
+`react-dom`, `matrix-js-sdk` and `livekit-client` itself, since the bundle leaves
+them external.
+
 ### Backend
 
 A docker compose file `docker-compose-dev.yml` is provided to start the