ui: fix Vercel build — explicit config, drop public flag, relax pnpm engine constraint

[suresh-hashicorp]

Description

Isolates the Vercel build fixes from #23707 into a standalone PR targeting main.

Three files changed, all under ui/:

ui/vercel.json

• Add $schema for editor/validator support
• Remove unsupported "public": true flag (caused Vercel deploy errors)
• Add explicit installCommand: pnpm install --no-frozen-lockfile
  — --no-frozen-lockfile avoids install failures when Vercel's Node environment
  produces a different lockfile hash than the one committed
• Add explicit buildCommand: make dist-vercel
• Add explicit outputDirectory: dist

ui/package.json

• Relax engines.pnpm from ">= 10.*" to ">= 9" so Vercel's bundled
  pnpm 9 satisfies the constraint without requiring a global install step

ui/README.md

• Add one-line note pointing to ./vercel.json for Vercel preview deployment config

Testing & Reproduction steps

Vercel preview deployment on a branch should now complete without:

• ERR_INVALID_ARG_TYPE / unsupported config key errors (public)
• ERR_PNPM_FROZEN_LOCKFILE failures
• pnpm engine version mismatch errors

Links

• Parent PR: migrate services, nodes, service instances, intentions and access control pages to new UI #23707

PR Checklist

• not a security concern
• updated test coverage (n/a — config-only change)
• external facing docs updated (n/a)
• appropriate backport labels added

[hc-github-team-consul-core]

📣 Hi @suresh-hashicorp! a backport is missing for this PR [23743] for versions [1.21,1.22,2.0] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>

[hc-github-team-consul-core]

📣 Hi @suresh-hashicorp! a backport is missing for this PR [23743] for versions [1.21,1.22] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>

[hc-github-team-consul-core]

📣 Hi @suresh-hashicorp! a backport is missing for this PR [23743] for versions [1.21,1.22] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>

[hc-github-team-consul-core]

📣 Hi @suresh-hashicorp! a backport is missing for this PR [23743] for versions [1.21,1.22] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>