Add Fleet Desktop.app to .gitignore

[lucasmrod]

Resolves #35006.
See #35006 (comment).

Output when running the dirty check on main:

make desktop-app-tar-gz
go run ./tools/desktop macos
{"level":"info","command":"/Users/lucas/go/bin/go build -o Fleet Desktop.app/Contents/MacOS/fleet-desktop_amd64 -ldflags -X=main.version= ./orbit/cmd/desktop","time":"2025-12-30T08:38:20-03:00","message":"Build fleet-desktop executable amd64"}
{"level":"info","command":"/Users/lucas/go/bin/go build -o Fleet Desktop.app/Contents/MacOS/fleet-desktop_arm64 -ldflags -X=main.version= ./orbit/cmd/desktop","time":"2025-12-30T08:38:23-03:00","message":"Build fleet-desktop executable arm64"}
{"level":"info","output":"On branch test-branch-dirty\nUntracked files:\n  (use \"git add <file>...\" to include in what will be committed)\n\tFleet Desktop.app/\n\nnothing added to commit but untracked files present (use \"git add\" to track)\n","time":"2025-12-30T08:38:26-03:00","message":"git status"}
Error: detected dirty executable: {Path:github.com/fleetdm/fleet/v4 Version:v4.43.5-0.20251230113816-9bae7b475999+dirty Sum: Replace:<nil>}
exit status 1
make: *** [desktop-app-tar-gz] Error 1

Output on this branch:

make desktop-app-tar-gz
go run ./tools/desktop macos
{"level":"info","command":"/Users/lucas/go/bin/go build -o Fleet Desktop.app/Contents/MacOS/fleet-desktop_amd64 -ldflags -X=main.version= ./orbit/cmd/desktop","time":"2025-12-30T08:39:43-03:00","message":"Build fleet-desktop executable amd64"}
{"level":"info","command":"/Users/lucas/go/bin/go build -o Fleet Desktop.app/Contents/MacOS/fleet-desktop_arm64 -ldflags -X=main.version= ./orbit/cmd/desktop","time":"2025-12-30T08:39:47-03:00","message":"Build fleet-desktop executable arm64"}
Generated desktop.app.tar.gz successfully.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.

fleetd/orbit/Fleet Desktop

• Verified that fleetd runs on macOS
• Verified auto-update works from the released version of component to the new version (see tools/tuf/test)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.82%. Comparing base (cc1485c) to head (2832135).
⚠️ Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #37711      +/-   ##
==========================================
- Coverage   65.88%   65.82%   -0.06%     
==========================================
  Files        2361     2362       +1     
  Lines      187298   187509     +211     
  Branches     7908     7908              
==========================================
+ Hits       123394   123437      +43     
- Misses      52622    52774     +152     
- Partials    11282    11298      +16     

Flag	Coverage Δ
backend	67.67% <ø> (-0.07%)	⬇️

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