website: Fix WASM build: remove missing matrix theme and fix App import (#2723)

Two compile errors broke the WASM build job.

## Description

- **`crates/story/src/embedded_themes.rs`**: Removed the `matrix` theme
entry — `themes/matrix.json` does not exist in the repository, causing a
compile-time `include_str!` failure.

- **`crates/story/src/stories/editor_story.rs`** (`#[cfg(target_family =
"wasm")]` module): Replaced unused `Context` import with `App`, which
was referenced in the `InputHighlighter::update` signature but missing
from the import list.

```diff
- use gpui::{Context, HighlightStyle, SharedString, Window};
+ use gpui::{App, HighlightStyle, SharedString, Window};
```

## Screenshot

N/A — build fix only.

## How to Test

```bash
cargo build -p gpui-component-story --target wasm32-unknown-unknown
```

## Checklist

- [ ] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [ ] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ ] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: huacnlee <5518+huacnlee@users.noreply.github.com>