Share Compute (mesh-llm) is unavailable in every official Linux build — release and canary

**Title:** Share Compute (`mesh-llm`) is unavailable in every official Linux build — release and canary

**Body:**

Share Compute fails on Linux with `Couldn't check shared compute: mesh-llm feature not enabled` (the stub error from `desktop/src-tauri/src/mesh_llm_stubs.rs`) on the current `apt`-installed `.deb` (v0.5.2).

Traced this to the release/canary CI definitions rather than a local config issue:

- `mesh-llm` is an opt-in Cargo feature (`desktop/src-tauri/Cargo.toml`): `default = ["system-keyring"]`, with `mesh-llm` requiring an explicit `--features mesh-llm` flag.
- `.github/workflows/release.yml`: only the macOS job (`runs-on: macos-latest`, line ~181) passes `--features mesh-llm`. `release-linux` (line ~624, `pnpm tauri build --verbose --ci --bundles deb,appimage ...`) does not. Neither does `release-windows`.
- `.github/workflows/linux-canary.yml` (line ~173) also builds without `--features mesh-llm`.

So there is currently no official Linux artifact — stable or canary — that ships Share Compute at all, even though the feature and its UI are fully present and reachable in the Linux build (it just always errors).

**Ask:** either add a Linux release/canary variant built with `--features mesh-llm` (behind a separate bundle name, like the mac path already does), or gate the Share Compute UI behind a build-time flag so Linux users don't hit a dead-end error screen for a feature their build can never support.

Environment: Ubuntu (ai-local), `buzz` 0.5.2 via apt, binary `/usr/bin/buzz-desktop`.