Build a second client without the embedded server

[sivert-io]

Replaces #389, which GitHub closed when its base branch was deleted on the
#388 merge. Same work, rebased onto main.

GRYT_VARIANT=slim produces a second build with the embedded server left out.
Measured on Windows, both packaged from this branch:

build	installer
yarn electron:build	126 MB
yarn electron:build:slim	92 MB

Electron's own runtime is about 90 MB of that 92, so the slim build is roughly
Gryt plus Chromium and not much else.

How it hangs together

The variant reaches three places that do not call each other, so it travels in
the environment:

• beforeBuild skips packaging the runtime. That is also most of why a slim
  build is quicker — packaging is the step that needs the server, worker and SFU
  checkouts.
• check-extra-resources stops demanding those two files, and still checks
  everything else. That script exists because electron-builder skips a missing
  extraResource silently, and this leans on exactly that behaviour.
• electron-builder.config.cjs reads electron-builder.yml and applies the
  three things YAML cannot branch on. The YAML is still where everything is
  written down.

Two of those three are about the release rather than the build: without distinct
artifact names the variants overwrite each other in one release, and without its
own update channel a slim install downloads the full installer next time and
quietly puts the 34 MB back.

What to look at

The updater. main.ts deliberately does not set autoUpdater.channel.
electron-builder bakes channel: slim into app-update.yml, which is the
build's own statement of which variant it is; assigning the property would
override that with a value inferred from a file being absent. The inference is
used only by channelYmlName(), which reads the yml before electron-updater
loads anything — so if it is ever wrong the result is "no update available"
rather than the wrong installer. Worth a second opinion.

Verified

Both variants packaged and launched on Windows against an isolated profile.
Slim produces Gryt-Chat-1.9.14-win-x64-slim.exe, slim.yml, and resources
with no embedded-server.tar.gz or embedded-native/; the default build still
produces latest.yml and both embedded resources. Both start, show a window and
a tray, and discover LAN servers. check-builder-config validates both configs
against electron-builder's schema and asserts the filter, the channel and the
artifact name.

The smoke test found one thing, fixed here: the startup log said "Embedded
server runtime ready" in a slim build, because prepareEmbeddedServerRuntime()
returns early when there is no archive and the line after it ran regardless.

Not verified: macOS and Linux. Signing and notarization differ, and the mac
path stages the native overlay separately.

Still to do, not in this PR

• The release workflow in the superproject needs a variant axis. Nothing
  publishes a slim artifact until then.
• The site needs the download tick, defaulting to slim.

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