chore: configure size-limit for bundle size CI

[masch]

Currently, the bundle size CI workflow (ci-bundle-size.yml) only outputs the total size of the web export directory to the GitHub Step Summary.

We should implement size-limit to:

1. Define limit rules per bundle path (e.g. apps/mobile/dist/**/*.js).
2. Show a diff/breakdown of bundle sizes.
3. Block PRs if they exceed specified limits.

To do this:

• Create a .size-limit.json at the project root or add a size-limit section to package.json.
• Switch to using bunx size-limit in the CI workflow.