Fix: enable transparent Tauri window

[MathCunha16]

Summary

Fixes a macOS release build failure: WebviewWindowBuilder::transparent(), used in recreate_splash_window (added to programmatically rebuild the splash window when reopening from the tray after the grace-period destroy), requires the transparent feature flag on the tauri crate. It was previously declared only in tauri.conf.json (which has its own schema with no such restriction), so the missing flag only surfaced once the splash window started being built via Rust code as well.

Related issue

Closes #

Type of change

• Bug fix
• New feature
• Refactor or maintenance
• Documentation
• Other (describe below)

How was this tested?

• Reproduced the failure locally by running the macOS build (npm run tauri build) and confirming the same E0599: no method named 'transparent' compile error.
• Added "transparent" to the tauri crate's feature list in Cargo.toml and reran the build, confirming it compiles successfully.
• Verified the splash window still renders correctly (transparent background, no decorations, always-on-top, centered) both on first launch and when recreated after reopening from the tray past the grace period.

Checklist

• I have read CONTRIBUTING.md.
• The change is focused and does not include unrelated refactors.
• I added or updated documentation when needed.
• I added or updated tests when the behavior requires it.
• I ran the relevant existing checks locally.
• I did not include secrets, credentials, personal data, or generated build artifacts.
• I understand the code I am submitting and reviewed all AI-assisted output, if any.
• I disclosed significant AI assistance that affected the design or implementation, if applicable.

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