ci(nightly): scope concurrency per branch

[loss-and-quick]

Summary

The Nightly workflow's concurrency group was the static nightly, shared across every branch:

concurrency:
  group: nightly
  cancel-in-progress: true

With cancel-in-progress: true, dispatching a nightly on one branch cancelled an in-flight nightly on another branch. Seen in practice: two manual workflow_dispatch runs 13 seconds apart on different branches (feat/proxy-mode at 11:51:36, feat/asset-auto-update at 11:51:49) — the second cancelled the first, leaving its module / desktop jobs cancelled on "Set up job".

Keying the group on github.ref scopes cancellation to a single branch: a newer run still supersedes an older run of the same branch (the intent of cancel-in-progress), but parallel branches no longer fight over one global slot. This matches the per-discriminator style already used in sync-bun-nix.yml (sync-bun-nix-${{ github.event.pull_request.number }}).

release.yml's global release group is intentionally left as-is — it's a deliberate single global lock (cancel-in-progress: false) so two releases never bump module.prop at once.

Affected layer

• frontend/ — React Web UI
• crates/ · src-tauri/ — Rust core / backend / Tauri desktop
• module/ — Android installable zip (thin launcher over the Rust daemon)
• scripts/ — build / release helpers
• CI / .github/
• Docs only

Verification

• actionlint (v1.7.12, the version pinned in lint-workflows.yml) clean on nightly.yml

Checklist

• Title is a scoped Conventional Commit; commits are logically split
• No build artifacts committed
• Generated frontend/src/generated/ not touched
• No user-visible strings changed
• No project-id renames

Notes for reviewers

Pure CI scoping fix; no build logic touched.