feat: support cross-platform dark mode on Windows and Linux

[mgaldamez]

Description

This PR resolves the limitation where --dark-mode initialization and dynamic theme synchronization (update_theme_mode) were restricted only to macOS. It extends support for dark theme synchronization and forced dark mode to Windows and Linux (tested and verified on Fedora 44) using Tauri's native, cross-platform theme API.

Changes

1. Unconditional tauri::Theme import: Removed macOS-only compilation guards (#[cfg(target_os = "macos")]) around Themeimports in bothwindow.rsandinvoke.rs`.
2. Cross-platform initialization: Updated window builder configuration on Windows/Linux to honor the dark_mode preference dynamically using .theme(theme) instead of forcing .theme(None).
3. Cross-platform dynamic setting: Updated the update_theme_mode command handler to call window.set_theme on all target platforms, eliminating OS-specific guards.
4. Documentation: Updated --dark-mode descriptions in the CLI parser configuration and user documentation (docs/cli-usage.md and docs/cli-usage_CN.md) to explicitly denote macOS, Windows, and Linux support.

Verification

• Tested and verified at runtime on Fedora 44 that launching the app with --dark-mode sets the window theme to dark correctly.
• All 12 Rust Cargo tests compiled and passed.
• All 193 Vitest CLI tests passed successfully.

Closes #1244

[tw93]

Thanks @mgaldamez, merged. Clean change: it mirrors the existing macOS theme logic onto Windows and Linux, regenerates dist/cli.js, and updates both English and Chinese docs. It'll ship in the next release.

One note for Linux users: dark mode there goes through WebKitGTK, so whether a page actually renders dark also depends on the WebKitGTK build honoring the window theme and the site implementing prefers-color-scheme: dark.