Version resolution should try local first, then fall back to remote

[sdairs]

Problem

When resolving a version spec for local server start -v <spec> (or local install), we currently make a remote lookup (GitHub API / builds.clickhouse.com) before checking whether the requested version is already installed locally. This means a fully-local request can still fail or hang on a remote call.

Desired behaviour

Always try to resolve locally first. Only fall back to remote lookup if no local match exists.

• -v 26.4 → look for any installed 26.4.x.y. If found, use it. Otherwise, look up remotely.
• -v 26.4.1.1 → look for an exact local match. If found, use it. Otherwise, look up remotely.
• -v 26.4.1 → same pattern: look for any installed 26.4.1.y. If found, use it. Otherwise, look up remotely.
• -v stable / -v lts / -v latest → these are channel/floating specs and should continue to consult the remote, since "what's stable today" is a remote concept.

Why

• Faster: no network round-trip when the bits are already on disk.
• Works offline: starting a server with an already-installed version shouldn't require internet.
• Predictable: if the user has the version locally, that's what they get.