Fix ubuntu-latest CI failure by provisioning Tauri Linux deps and restoring required icon asset

[Copilot]

The test (ubuntu-latest) Actions job failed in src-tauri because required Linux system libraries were missing for glib/gobject/gio pkg-config resolution. The same path also required src-tauri/icons/icon.png, which was absent from the repository checkout.

• Ubuntu CI system dependency provisioning

  • Added an Ubuntu-only install step in .github/workflows/ci.yml before Rust checks.
  • Installs the Tauri/Linux native dependencies needed by cargo test/cargo clippy in src-tauri.

• Tauri bundle asset completeness

  • Added src-tauri/icons/icon.png so tauri::generate_context!() can load the configured bundle icon during compile/test paths.

Example workflow addition:

- name: Install Linux system dependencies
  if: matrix.os == 'ubuntu-latest'
  run: |
    sudo apt-get update
    sudo apt-get install -y \
      libglib2.0-dev \
      libgtk-3-dev \
      libwebkit2gtk-4.1-dev \
      libayatana-appindicator3-dev \
      librsvg2-dev

[Knownassa]

Approved: restores required Linux Tauri dependencies and tracked Windows/Linux icon resources.