deck: add base path plumbing and helper APIs

[tsj-30]

This change teaches Deck about a configurable base path so it can be served from a sub‑URL without breaking static assets, APIs, or Spyglass. It introduces the --base-path flag, wraps the HTTP mux so requests are stripped/prefixed appropriately, extends template helpers to emit base-aware links, and adds deckURL utilities for the front-end.
This fixes #376.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tsj-30
Once this PR has been reviewed and has the lgtm label, please assign michelle192837 for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• cmd/deck/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @tsj-30. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[netlify]

✅ Deploy Preview for k8s-prow ready!

Name	Link
🔨 Latest commit	f90bdd0
🔍 Latest deploy log	https://app.netlify.com/projects/k8s-prow/deploys/693b7e2af8ae6100080c1fe2
😎 Deploy Preview	https://deploy-preview-576--k8s-prow.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[tsj-30]

CC: @petr-muller , @matthyx
As suggested by Petr, I’ll break this down and raise smaller PRs to fix #376.

[Amulyam24]

/ok-to-test

[k8s-ci-robot]

@tsj-30: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-prow-verify-lint	f90bdd0	link	true	/test pull-prow-verify-lint

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.