feat: installer and downloads pages

[evereq]

Description

1. Make sure GitHub action runs on "main" branch and build / publish Desktop installers for the following platforms:

• Mac - arm64 (default if not detected) / x86
• Windows - x86 (default if not detected) / arm64
• Linux - x86 / arm64

I can recommend to use following Action runners to save on costs and improve performance:

• for Windows arm64, use GitHub windows-11-arm runner
• For Linux x64, use Ubicloud runners, e.g. ubicloud-standard-8
• For Linux arm64, use Ubicloud runners, e.g. ubicloud-standard-8-arm
• For Mac, use https://cirrus-ci.org/guide/macOS, e.g. ghcr.io/cirruslabs/macos-runner:tahoe
• For Windows x64, use either GitHub or https://www.warpbuild.com (note: this one get expensive fast, think about option of self-hosting Win x64 runners, easy to setup on some "home" server / laptop usually in few hours)

2. The setup files should be published into "Assets" in the releases - https://github.com/Light-Heart-Labs/DreamServer/releases/tag/v2.0.0

3. We need to support "pre-releases" and "stable" releases

4. We need to build downloads blocks in the https://get.dreamserver.ai page. Note at https://lightheartlabs.io, scroll the the button "Get Started" -> which redirects to https://get.dreamserver.ai.

How to organize this page:

• it should auto-detect target OS / architecture (x64 vs arm64) and allow to just click download to get it.
• however it will be great to allow user to select x64 vs arm64 manually too
• there should be selected "stable" option, but we need to allow user to select "pre-release"

Use Case

We need a quick way for people to try Dream Server - for those who don't have time or don't know how to install from CLI / clone repo, etc.

Proposed Solution

No response

Alternatives Considered

No response

[Lightheartdevs]

Triage note: this is still valid, but it is really three separable deliverables and should probably be split before implementation.

1. Release artifact pipeline: build/sign/package platform installers for macOS, Windows, and Linux across x64/arm64 where supported.
2. Release publishing: attach those artifacts to stable and prerelease GitHub releases with checksums and clear channel metadata.
3. Downloads page: detect OS/arch, allow manual override, and expose stable/prerelease channels without hiding CLI install paths.

The current repo has release-note automation and installer simulation/testing, but not a complete cross-platform desktop-installer release pipeline. A good first PR here would add the release asset manifest/checksum shape or one platform's packaged artifact path, not the whole downloads experience at once.