enable cloud sign-in in mirror builds and add installation smoke tests

[TonybynMp4]

The mirror's .deb failed to launch on a clean Debian 13 (libasound.so.2 missing), cloud sign-in and T3 Connect were compiled out, a partial build could still publish an update feed missing an architecture or package type, and the docs still claimed upstream ships no arm64 Linux build.

• Package dependencies: in electron-builder, setting depends replaces the default list, so the metadata patch now lists the defaults plus libasound2/libgbm1 (deb) and (alsa-lib or libasound2)/(mesa-libgbm or libgbm1) (rpm).
• Install smoke test: a new install job installs each package (deb/rpm × x64/arm64) into a clean debian:13 / fedora:43 container. It fails the release if ldd finds unresolved libraries or resources/package-type is wrong, and publish now waits for it.
• Update feed: merge-linux-update-manifests.py refuses to merge unless both the x64 and arm64 feeds exist and each contains both deb and rpm manifests.
• T3 Connect: the build bakes in upstream's public Clerk/relay config (the same values shipped in upstream's release artifacts). Every "disabled" mention is removed from MIRROR.md and the release notes.
• Unofficial labelling: package and AppStream descriptions now say this is an unofficial community build.
• Docs: MIRROR.md now says upstream ships x64 and arm64 AppImages.

Tested by installing the v0.0.42 x64 packages in real containers, with post-install scripts running:

	released	with new depends
deb, Debian 13	fails: libasound.so.2 => not found	passes
rpm, Fedora 43	passes only because weak deps pull in alsa-lib/mesa-libgbm; with install_weak_deps=False neither is installed	passes

arm64 wasn't tested locally; the CI job covers it.

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