macOS: uninstall helper for Application Support data (depends on #825)

[AgreeDK]

Dragging OpenSAK.app to the Trash removes the application bundle but leaves
~/Library/Application Support/opensak/ (databases, config) behind — which is
the correct default per our "opt-in data removal only" principle (see
Roadmap item 1), but most users never clean these up manually if they
genuinely want to.

This was blocked by #825 (settings_store.py branching on os.name == "posix",
which is true on both Linux and macOS, so OpenSAK was actually writing to
~/.config/opensak and ~/.local/share/opensak on Mac instead of the
documented ~/Library/Application Support/opensak/). Now that #825 is fixed,
this can proceed against the correct, verified path.

Goal

A small, separate, optional uninstall helper — downloadable from opensak.com,
not bundled inside the app — that:

1. Removes the app bundle itself (/Applications/OpenSAK.app), or confirms
   the user already trashed it.
2. Explicitly asks (default: No) whether Application Support data
   (databases, settings) should also be removed.
3. Only touches the documented path — no guessing, no wildcard deletion.

Suggested form

A minimal signed shell script (.command file) is the lowest-effort option;
a tiny "OpenSAK Uninstaller.app" is friendlier for non-terminal users but
needs its own (lightweight) signing/notarization pass. Recommend starting
with the .command script — cheap to ship, can be upgraded to a proper .app
later without changing the underlying logic.

Acceptance criteria

• Manual verification against macOS: fix data-path bug in settings_store.py (writes to wrong directory) #825's fix: fresh install on a real Mac
  lands in ~/Library/Application Support/opensak/ (per macOS: fix data-path bug in settings_store.py (writes to wrong directory) #825's own
  verification step, if not already closed by then).
• Helper removes the app bundle (or no-ops gracefully if already trashed).
• Helper prompts before touching Application Support data; default is
  "keep data."
• Helper never touches any path outside the documented Application
  Support location (equivalent of Linux AppImage: in-app uninstall #837's home-directory safety guard —
  reuse that pattern/tests if the logic translates).
• Signed appropriately so Gatekeeper doesn't block it outright.
• Documented on opensak.com's uninstall guide.

Out of scope

• Auto-upgrade (Sparkle) — tracked separately, not a dependency of this issue.
• Any change to the DMG install flow itself.

Related

• macOS: fix data-path bug in settings_store.py (writes to wrong directory) #825 (data-path fix — prerequisite)
• [Epic] Installation, Uninstallation & Auto-Upgrade (Roadmap #1) #824/Linux AppImage: in-app uninstall #837 (Linux equivalent — similar safety-guard pattern worth reusing)