build: sync platform icons in bash Meson wrapper for non-Windows CI

[themuffinator]

Motivation

• Ensure non-Windows Meson wrapper performs the same icon staging as the PowerShell wrapper to avoid missing icon assets causing Linux/macOS nightly failures (CI run 24158948319).

Description

• Add sync_icons_script pointing to tools/build/sync_icons.py in tools/build/meson_setup.sh.
• Invoke the icon sync step via the resolved Python interpreter for setup, compile, and install commands unless OPENQ4_SKIP_ICON_SYNC=1, and fail with a clear error if the script is missing.
• Aligns bash wrapper behavior with tools/build/meson_setup.ps1 to remove a platform discrepancy that caused staged payload validation to fail.

Testing

• Ran shell syntax check bash -n tools/build/meson_setup.sh, which succeeded.
• Ran python3 tools/build/sync_icons.py --source-root ., which completed successfully and reported generated/processed icons.
• Attempted full Meson setup via bash tools/build/meson_setup.sh setup --wipe builddir ..., which could not complete because meson is not installed in this environment and package installation is blocked by the network/proxy, so the Meson build step could not be validated here.

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Codex Task