fix: submit project IDs through a form

[floze-the-genius]

Summary

• submit the project ID through a semantic form so Enter and the Go button use the same path
• explicitly associate the label with the input and remove the extra wrapper tab stop
• mark the field required and request a numeric keyboard while preserving the existing digit filtering

Closes #70.

Verification

• npm ci with Node.js 22.23.1 / npm 10.9.8
• npm run format:check
• parsed and compiled ProjectInput.vue with @vue/compiler-sfc; verified the generated render includes the submit handler, prevent modifier, label target, and required input
• direct npm run build and npm test -- --run on a clean checkout reach the existing prerequisite failure for untracked generated WASM/JS modules; the repository CI creates those modules with build.sh -Wpz before its integration test

[pufferfish101007]

The "verification" that your LLM (I presume) claims to have run is neither sufficient nor particuarly relevant. For such a simple change, I would expect a human to actually try it out and check that the behaviour matches what you'd expect; if you're not going to make an effort to do that then I will not make an effort to review this.

If you'd like to engage properly with open-source then I will be happy to engage in return. My apologies if a) you just write in a very LLM-like style or b) if you thought that this sort of pull request would be ok due to my lack of mentioning it in the contribution guidelines.