[bug] Absolute frontendDist path does not work on Windows

[aspecter]

Describe the bug

Using absolute frontendDist path on Windows builds won't embed the assets, instead just links them: file:///D:/some/path/to/dist.

Reproduction

1. Set the frontendDist on Windows to some absolute path, like "frontendDist": "D:\\some\\path\\to\\dist".
2. Check the parsed config object.
3. Note that frontend_dist is Some(Url(Url { scheme: "d", cannot_be_a_base: true, username: "", password: None, host: None, port: None, path: "\\some\\path\\to\\dist", query: None, fragment: None })) instead of Some(Directory("D:\\some\\path\\to\\dist")).
4. Run the built application and see the error in the console.

Expected behavior

Absolute path based assets should be embedded properly on Windows, like on Linux and macOS.

Full tauri info output

npm run tauri info

> tauri-app@0.1.0 tauri
> tauri info


[✔] Environment
    - OS: Mac OS 15.5.0 arm64 (X64)
    ✔ Xcode Command Line Tools: installed
    ✔ rustc: 1.87.0 (17067e9ac 2025-05-09)
    ✔ cargo: 1.87.0 (99624be96 2025-05-06)
    ✔ rustup: 1.28.2 (2025-04-28)
    ✔ Rust toolchain: stable-aarch64-apple-darwin (default)
    - node: 22.13.1
    - npm: 10.9.2
    - deno: deno 2.4.3

[-] Packages
    - tauri 🦀: 2.7.0
    - tauri-build 🦀: 2.3.1
    - wry 🦀: 0.52.1
    - tao 🦀: 0.34.0
    - @tauri-apps/api : not installed!
    - @tauri-apps/cli : 2.7.1

[-] Plugins
    - tauri-plugin-opener 🦀: 2.4.0
    - @tauri-apps/plugin-opener : not installed!

[-] App
    - build-type: bundle
    - CSP: unset
    - frontendDist: d:\some\folder

Stack trace

Additional context

This issue came up when we created a Tauri wrapper package to reuse across multiple projects.
The wrapper package uses a build script to utilise tauri build, and those scripts are creating build specific tauri.conf.json files. The simplest approach was to use absolute paths, but it did not work on Windows, so we switched to relative paths. We also wanted to use our wrapper as a globally installed package on Github actions, where workspaces are on D: and global npm packages are on C:. As a result, our “relative” paths resolved to D:\some\folder\**\* based on the current working directory D:\some\folder, while the global package was located at C:\Users\[Username]\AppData\Roaming\npm\node_modules\our_package.

[FabianLars]

as a workaround until this is resolved, do unc paths work? eg \\?\D:\some\path (or "\\\\?\\D:\\some\\path" escaped)

[aspecter]

It works, thanks! We will stay with this workaround until fix.
Alternative workaround is to set the global npm package directory to the same drive and use relative paths.

[Legend-Master]

Seems like a similar case as tauri-apps/plugins-workspace#1710 not sure if we could assume single letter URL scheme as Windows path in this case though