Migrate StandUp desktop app to Tauri 2

[pyruth]

What changed

• Migrate StandUp from Electron to a lightweight Tauri 2 desktop application for Windows x64 and Apple Silicon macOS.
• Add the native tray/menu-bar timer, launch-at-login support, monitor and position selection, offline sounds, and secure custom GIF import/reset.
• Package branded Windows NSIS and macOS DMG artifacts through native CI runners.
• Replace the reused reminder WebView with a fresh, click-through popup for every reminder.
• Add a renderer-to-Rust readiness checkpoint so the popup is shown only after the GIF loads and decodes.
• Add default-animation fallback, unique reminder sessions, and loading/visibility watchdogs.
• Preserve the recently merged platform-specific tray icon assets and behavior.

Why

The Electron build was large and had native dependency packaging issues. The persistent reminder popup could also become stuck: Rust marked it visible before positioning, JavaScript initialization, GIF decoding, and window display completed. Any failure left the in-memory flag set, causing later previews and scheduled reminders to be skipped until restart.

User impact

StandUp remains private and offline while producing a much smaller installer. Reminder previews and scheduled reminders now recover automatically from GIF, WebView, monitor, or timing failures. A stale timer cannot close a newer reminder, and invalid custom GIF playback falls back to the bundled animation.

Validation

• 19 TypeScript tests passed.
• TypeScript type checking passed.
• 15 Rust tests passed.
• Strict Rust Clippy validation passed with warnings denied.
• Production renderer build passed.
• Windows x64 NSIS installer built successfully as StandUp 2.0.3.
• Windows executable verified as GUI subsystem with no console window.

[pyruth]

Closing because v2 is the product's default Tauri branch; main intentionally retains the Electron version.