Add CI workflow to build Tauri app artifact for PRs and master

[Copilot]

This adds a dedicated GitHub Actions build workflow for the Tauri desktop app so PRs and master produce a reusable build artifact. It sets up the Linux build environment, uses the existing repo build entrypoint, and uploads the built app binary for later release automation.

• Workflow scope

  • Adds .github/workflows/build.yml
  • Triggers on all pull requests and pushes to master
  • Keeps permissions minimal with read-only contents access

• Build job

  • Checks out the repo with submodules
  • Sets up Node with Yarn cache and stable Rust
  • Installs just, tauri-cli, the repo-specific libnotmuch-dev, and the Tauri Linux prerequisite packages from the upstream docs

• Artifact output

  • Builds the app from the repo root using the existing just build-debug command
  • Uploads the confirmed built desktop binary as a named workflow artifact from target/debug/amail

• Build invocation

  - name: Build Tauri app
    run: just build-debug

[OJFord]

@copilot

[OJFord]

@copilot

[Copilot]

@copilot

Addressed in 9bb403c: the workflow now uses the Tauri Linux prerequisite list with an inline source comment, keeps just/libnotmuch-dev as repo-specific extras, and drops JUST_UNSTABLE from the build step.