feat(cli): rename the server start command from `visionset ui` to `visionset server`

[JArmandoAnaya]

Motivation

visionset ui starts the FastAPI server; the web app is one client of it, alongside
REST, the SDK and MCP. The command name describes one consumer of the process, not the
process. visionset server names what the command actually does, and keeps the CLI
honest as more clients (Qt6 desktop, agents) land against the same API.

Decisions (settled here, not in the PR)

• Clean rename, no alias. We are pre-PyPI; visionset ui is removed outright.
  Running it must fail with Typer's normal unknown-command error — no hidden
  deprecated alias, no custom "did you mean" shim.
• Behaviour is untouched. Same server, same /app mount, same refusal outside a
  workspace ("one sentence and exit 1"), same flags, same VISIONSET_UI_SESSION
  semantics.

Scope

• CLI: rename the Typer command and its module/tests as needed
• Every invocation in the repo: docs (README.md, docs/install.md,
  docs/tutorial.md, docs/cli.md, docs/auth.md, docs/mcp*.md,
  CONTRIBUTING.md, docs/README.md — and any other hit), examples
  (examples/http_end_to_end.py starts the command for real), Docker compose /
  entrypoints, scripts, tests, AGENTS.md
• Prose that names the command ("visionset ui run outside one refuses…") updated
  to the new name

Out of scope

• VISIONSET_UI_SESSION and any other *_UI_* identifier that is not the CLI
  command — renaming env vars is a separate decision (follow-up issue if wanted)
• The /app route and _static/ bundling — unchanged