feat(SquirrelPanel): add right-click menu to delete a candidate

[sxin0]

What

Right-clicking a candidate opens a context menu with a single item that
forgets that candidate.

This wires up a librime API that Squirrel has never called:

RimeApi.delete_candidate_on_current_page(session, index)

Weasel has exposed the same operation as Shift+Delete for a long time;
Squirrel had no entry point for it at all.

Implementation notes

• SquirrelInputController.deleteCandidate(_:) is deliberately symmetric
  with the existing selectCandidate(_:).
• Hit testing reuses SquirrelView.click(at:), the same path left-click
  selection already goes through.
• Window level. The panel is created at CGShieldingWindowLevel(),
  which is far above the level AppKit assigns to pop-up menus, so the
  menu would be drawn behind the candidate panel. The panel is lowered
  to NSWindow.Level.popUpMenu - 1 for the duration of menu tracking and
  restored afterwards. 100 is still above ordinary application windows,
  so the panel is not covered by the client app while the menu is open.
• It deliberately does not call NSApp.activate. Activating would
  pull focus away from the client app and break the input session.
• Adds one localized string with zh-Hans / zh-Hant translations.

Only deletion is implemented. There is no pin-to-top counterpart because
librime exposes no such API — candidate ordering is decided by translator
weights and user-dictionary learning, and a frontend cannot override it.

Testing

This has not been built or run. It was written on a machine with no
Xcode installed, so verification stopped at swiftc -parse. Two things
need checking on a real build before this is merged:

1. Whether NSMenu.popUp(positioning:at:in:) tracks correctly from a
   non-activating panel in an LSUIElement app.
2. Whether .rightMouseDown reaches the panel's sendEvent (left-click
   already does, so this is likely but unverified).

Flagging this explicitly rather than letting a reviewer assume it was
smoke-tested.

[LEOYoon-Tsaw]

Mark 一下，有空我測測，這個有意思