docs: load the hub-served client script for manual injection

[MFA-G]

Description

The manual injection docs still tell users to import @vitejs/devtools/client/inject (and the -passive / -hidden variants). #529 removed those exports, so following the guide fails at module resolution.

The client is now served by the dev server at /__devtools/embedded.js. This PR points both pages at it:

• Guide → Projects without an HTML entry: one dev-only snippet that appends a module script for /__devtools/embedded.js, built from the dev server's origin. The three per-visibility imports are gone. The client reads embeddedVisibility from the server, so one entry covers all three modes.
• Kit → Client Script & Client Context: the client script description, the "How injection works" paragraph and sequence diagram (inline bootstrap → embedded.js, no virtual module), the iframe note (the [VITE DEVTOOLS] Skipping in iframe log no longer exists), and the troubleshooting fix and symptom.

The URL uses new URL(import.meta.url).origin because a root-relative /__devtools/embedded.js resolves against the page origin. On a backend-rendered page that origin is the backend, not Vite. new URL('/__devtools/…', import.meta.url) doesn't work either, because Vite rewrites it to /@fs/__devtools/embedded.js (404).

Linked Issues

Closes #589

Additional context

I checked the snippet with the published Vite 8.3.1 and this branch's build. The HTML came from a separate plain HTTP server on another port, and main.js was loaded from the Vite dev server (devtools: { enabled: true }). In headless Chrome, <devframes-dock-embedded> mounted and the hub client context was published. This happened with both the default clientAuth and clientAuth: false. The old import still fails with "./client/inject" is not exported.

pnpm exec eslint on the two files and pnpm -C docs run docs:build pass.