`gpui_web` currently embeds IBM Plex Sans and Lilex into every browser
application during platform construction. Applications such as Delta
already provide their own fonts, so those binaries contain overlapping
font bundles and register the same faces twice.

This change starts the web platform with an empty font database,
matching the ownership boundary used by native applications: the
application selects and registers its fonts before opening a window.
Delta already follows that sequence. GPUI's browser-capable examples now
load the former eight-face bundle explicitly through shared example
support, while the standalone `hello_web` example demonstrates
registering a minimal application font directly.

Testing performed:

- `cargo check -p gpui --examples --target wasm32-unknown-unknown`
- `cargo check -p gpui --examples`
- `cargo check -p gpui_web --target wasm32-unknown-unknown`
- `cargo check --manifest-path
crates/gpui_web/examples/hello_web/Cargo.toml --target
wasm32-unknown-unknown`
- `./script/clippy -p gpui_web --target wasm32-unknown-unknown`
- `cargo fmt -p gpui -p gpui_web --check`

Release Notes:

- N/A