Properly declare inputs and output for buildClient

[Gold856]

Description

This allows Gradle to skip building the UI if none of the files in photon-client changed, which allows us to make the JAR task depend on the UI build task without wasting time rebuilding unnecessarily, which means building the UI no longer needs to be a separate step.

Fixes #2085.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2025.3.2
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added

[Gold856]

Needs testing. Also, since you don't to run the task separately anymore, should we remove https://docs.photonvision.org/en/v2025.3.2/docs/contributing/building-photon.html#build-and-copy-ui-to-java-source?

[samfreund]

LGTM, lemme remove reqs for those checks and do another test. Then I'll merge it.