Add CI checks for all example crates (wasm32 target)

[niklabh]

Description

The current CI pipeline (.github/workflows/ci.yml) only checks oxide-browser and hello-oxide. Several other example crates are missing from CI:

• examples/audio-player
• examples/timer-demo
• examples/index
• examples/fullstack-notes/frontend (wasm32 target)
• examples/fullstack-notes/backend (native target)

If any of these examples break due to SDK changes, we won't know until someone tries to build them manually.

What to do

1. Add cargo check --target wasm32-unknown-unknown for each WASM example crate to the check-sdk job
2. Add cargo check (native target) for the fullstack-notes/backend crate
3. Optionally, add cargo build for examples to ensure they produce valid .wasm output

Files involved

• .github/workflows/ci.yml

Difficulty

Beginner — just add a few lines to the existing CI workflow. Great way to learn GitHub Actions!