fix(filetree): use forward-slash path separator for git diff paths (#121)

[Booyaka101]

Closes #121.

What this fixes

On native Windows, the file tree pane shows every file as a flat list at the root level — no directory hierarchy is rendered. Reported by @abdmoh123 in #121, with the symptom verified on multiple Windows machines and confirmed working on Linux + WSL.

Root cause

buildFullFileTree (and collapseTree) split and join paths using os.PathSeparator and filepath.{Dir,Join}. git diff always emits forward-slash paths regardless of host OS, but on Windows os.PathSeparator == '\'. So:

strings.Split("graphql-server/tests", string(os.PathSeparator))
// Linux:   ["graphql-server", "tests"]   ← splits correctly
// Windows: ["graphql-server/tests"]      ← single element, no split

Every file gets walked as a single path component, never matches an existing directory in the tree, and ends up as a top-level file child of the root. Hence the flat list @abdmoh123 saw.

Fix

Swap to the path package (forward-slash, OS-agnostic) for everything that handles git-supplied paths. Six call sites changed in pkg/ui/panes/filetree/filetree.go:

Before	After
filepath.Dir(name)	path.Dir(name)
string(os.PathSeparator) (×3)	"/"
filepath.Join(...) (×3)	path.Join(...)

Imports trimmed: drop "os" and "path/filepath", add "path". Net: +13 / −9 lines, single file.

Verification

Empirically tested both platforms before opening this PR — the existing test suite is the regression test:

Platform	Pre-fix	Post-fix
Windows (Go 1.26.2 native)	TestBuildFullFileTree fails: expected 5 nodes, but got 3	All tests pass
Linux (golang:1.26 Docker container)	All tests pass	All tests pass — byte-for-byte identical

Linux behavior is unchanged because os.PathSeparator == '/' on Linux already, and path.{Dir,Join} produce identical output to filepath.{Dir,Join} for forward-slash inputs. The fix only affects Windows code paths where the previous behavior was already broken.

TestBuildFullFileTree, TestCollapseTree, TestUncollapsableTree, and TestCloseDirsBelowDepth* all flip from failing to passing on Windows under the fix; no new tests were needed.

Side benefits beyond the file-tree rendering

DirNode.FullPath is also used to:

• Match against paths in SetCursorByPath (filetree.go:188) — used for cursor positioning. Pre-fix on Windows, paths stored as "a\b" would never match git-supplied "a/b". Post-fix, they match.
• Pass to SetDirPatch in tui.go:1349 — the dir-patch viewer for browsing all changes under a directory. Same fix applies.
• Get copied to clipboard via CurrNodePath. Post-fix, copied paths use / everywhere, which is what users will paste back into git commands regardless of host OS.

So the fix incidentally repairs cursor positioning and the dir-patch view on Windows alongside the visible tree rendering.

Credit

Original report and reproduction across multiple Windows machines and config states by @abdmoh123 in #121. They isolated it to native Windows (Linux and WSL work fine), tested with multiple terminal emulators, and confirmed the issue reproduces with default config — saving a lot of triage work.