chore: Add CI guard to keep tauri.conf.json version in sync with Cargo.toml

[amiable-dev]

Description

tauri.conf.json and the workspace Cargo.toml both declare a version string, but they are updated independently. This manual process already caused version drift once (#470, where tauri.conf.json was stuck at 4.26.6 while Cargo.toml was at 4.37.0). It was fixed in #513 but can regress.

Add an automated guard — either a CI check or a prebuild script — that fails if tauri.conf.json's version field differs from the workspace Cargo.toml [workspace.package].version.

Options

1. CI script (preferred): A step in the GitHub Actions workflow that parses both files and fails with a clear message if they differ.
2. Prebuild script: A beforeBuildCommand or npm script that checks version consistency before each build.
3. Cargo xtask: A workspace-level task (cargo xtask check-versions) that can be run locally and in CI.

Acceptance Criteria

• Build or CI fails with a clear error message when versions diverge
• The check compares Cargo.toml [workspace.package].version with tauri.conf.json version
• Documented in CLAUDE.md or CONTRIBUTING.md so contributors know the constraint

Context

Flagged during PR #513 review.