fix(desktop): strip Windows extended-length prefix before git clone (#3707)

[iroiro147]

Fixes #3707.

Root cause

When the Windows desktop app's repos root is canonicalized, Path::canonicalize() returns a verbatim extended-length path (\\?\C:\...). In clone_project_repository_blocking, that canonical repos_root flows through repos_root.join(repo_name) and into git clone <url> <destination> — and Git for Windows rejects the verbatim prefix with fatal: could not create work tree dir '\\?\C:\...': Invalid argument, even with core.longpaths=true.

The canonical form is the correct internal representation (the containment check against repos_root legitimate on it); it simply isn't a valid argument for the external Git CLI.

Fix

Add path_for_external_command(), which strips the \\?\<DRIVE>:\ prefix on Windows (cfg(windows) branch) and passes through unchanged elsewhere. Used exactly once, at the CLI-argument boundary. Everything else — containment checks, align_unborn_head_branch, .git inspection, the path field returned to the frontend — continues to see the canonical form, so no security or behavior invariants shift.

Test coverage

• clone_args_strip_verbatim_prefix_from_destination — the regression test the reporter asked for. On Windows CI it validates the strip; on other hosts it validates the pass-through while still exercising cfg guard logic. (The clone-destination assembly itself is cfg-agnostic and covered by existing tests; the only Windows-specific branch is the prefix strip.)
• path_for_external_command_leaves_ordinary_absolute_paths_alone

Verification performed

• cargo check --lib on macOS (cfg(not(windows)) branch): clean
• rustc --edition 2021 --crate-type lib --target x86_64-pc-windows-msvc on an extracted copy of the helper (cfg(windows) branch): clean. This caught a real bug during review — the initial draft used prefix_component.kind as if it were a field; it's a method (prefix_component.kind()). The full cross-target cargo check can't complete on macOS because some transitive deps need a Windows C toolchain (ring); the isolated-rustc route exercises the exact production code under the Windows cfg without the C-dep surface.
• cargo test --lib project_git_workflow::tests: 14/14 pass (12 prior + 2 new)
• cargo clippy --lib: zero warnings

[Inovvia]

@iroiro147 I cloned and built your branch and i was able to clone!

[iroiro147]

Thanks for the independent verification, @Inovvia — glad the clone path works end-to-end on a real Windows environment. That was the missing confirmation from #3707.