feat: add Linux platform support

[bet4it]

Summary

This PR adds comprehensive Linux platform support (Wayland and X11), enabling FlashVoice to run smoothly across modern Linux desktop environments.

Key Changes:

1. Window Management & Floating Indicator:

   • Fixed Wayland compatibility in tauri-plugin-windows, preserving the pill capsule shape and preventing window decorations from disrupting indicator layout.
   • Configured set_focusable(false) for the floating indicator window so clicking it does not steal keyboard focus from target input fields.
   • Implemented custom self-drawn titlebar for Linux desktop windows.

2. Global Shortcuts:

   • Implemented Linux global shortcut capture via handy-keys in tauri-plugin-keyhook, supporting push-to-talk and toggle-to-talk without stealing key input.

3. Text Injection (Auto-Type):

   • Added Linux implementation to tauri-plugin-textinject using ydotool and wl-copy/clipboard injection.
   • Safely saves and restores original clipboard contents in the background after paste execution.

4. Tray & Audio Playback:

   • Switched to full-color tray icon on Linux for optimal panel visibility.
   • Fixed audio playback on Linux by converting local recording paths to object blob URLs in WebKitGTK.
   • Wired Linux speaker input stubs and resolved unused variable warnings for clean builds.

Verification

• pnpm lint
• pnpm type-check
• pnpm test
• pnpm build
• pnpm rust:fmt:check
• pnpm rust:clippy
• pnpm rust:test
• Relevant Rust checks or tests were run when Rust code changed

Manual verification conducted on Linux (NixOS, Linux 6.12, GNOME 47 on Wayland):

• Tested push-to-talk and toggle-to-talk recording with both mouse and keyboard combinations.
• Tested automatic text injection into text editors, browsers, and terminal windows.
• Tested floating indicator window dragging, animations, and focus preservation.
• Tested system tray menu actions and recorded audio playback.

Risks and impact

• Compatibility: All new logic is guarded behind #[cfg(target_os = "linux")] or runtime platform checks (isLinux). macOS and Windows execution paths remain completely untouched.
• Performance: Key injection is lightweight; clipboard restoration is handled asynchronously in a background thread to prevent blocking the UI.
• Privacy: None. All speech recognition, audio caching, and text injection occur entirely on-device.

Checklist

• This pull request has one clear objective.
• Complex logic has clear English comments, and important boundaries have actionable logs.
• No API keys, tokens, signing material, personal data, build output, or assets with unclear licenses are included.
• User-visible changes include appropriate tests, documentation, or screenshots.