feat: pre-release update channel + automated pre-releases from main - #159

What & why

Adds a Pre-release update channel so every change merged to main is automatically shipped as a signed pre-release, and users can opt into early builds from Settings → Software Updates. The normal stable flow (release-please) is unchanged. See radiant-enchanting-sundae plan.

How it works

• Channel delivery via a header, not a Rust rewrite. Tauri can't template a channel into a static endpoint URL and JS check() can't set endpoints, so the app sends X-Mcpmux-Channel: stable|prerelease and a resolver at api.mcpmux.com/v1/update/latest.json returns the matching release's latest.json. This keeps the proven JS check/install path 100% intact (a broken updater can't fix itself) and yields per-channel update analytics. The GitHub /releases/latest URL stays as a fallback endpoint, so stable updates keep working even if the resolver is down or not yet deployed.
• Per-version pre-releases X.Y.Z-pre.<run_number>, where X.Y.Z is release-please's pending next version (read from its bot-branch manifest). SemVer ordering holds: 0.3.0 < 0.4.0-pre.318 < 0.4.0.
• Promote = both paths. Primary: merge the release-please PR (clean stable cut). Escape hatch: promote.yml rebuilds a chosen -pre.N commit as signed stable.
• release-please stays the changelog/version source of truth (manifest mode → v*-pre.* tags don't interfere).

Changes

App

• commands/settings.rs: get_update_channel / set_update_channel (key updates.channel, default stable) + unit tests; registered in lib.rs.
• lib/updates.ts: channel helper that sends the header; wired into both check() sites (App.tsx launch auto-check, UpdateChecker.tsx manual).
• UpdateChecker.tsx: Stable / Pre-release selector.
• tauri.conf.json: resolver endpoint + GitHub stable fallback.

CI

• release.yml: gated prerelease jobs (pre-create draft → build by id → publish) — race-free, like the stable flow.
• build-tauri.yml: reusable build shared by stable / pre-release / promote.
• promote.yml: workflow_dispatch promote-by-tag.
• scripts/set-version.mjs: stamps the version onto the 3 release-please-tracked files.

Tests / validation

• ✅ cargo clippy --workspace -- -D warnings, cargo fmt --check
• ✅ Rust unit tests (commands::settings, incl. new normalize_channel)
• ✅ pnpm typecheck, ESLint (0 errors), full TS suite (197 + 7 new)
• ✅ Workflow YAML parses; job graph verified

Companion PR (separate repo)

The resolver route (GET /v1/update/latest.json) ships in mcpmux.serverhub.api. After merging it: deploy the worker and wrangler secret put GITHUB_TOKEN (recommended for API rate limits). Until then, the fallback endpoint serves stable.