fix(macos): fix clipboard copy failing from tray and GUI

[Mitnitsky]

Problem

Copying screenshots to clipboard on macOS was broken in two ways:

1. 3-second delay + clipboard empty: copyToClipboard() created a
   KDSingleApplication instance and called isPrimaryInstance(), which
   always returned false because the daemon already holds the socket.
   This triggered IPC message delivery instead of using the in-process
   daemon directly. The IPC timed out after ~3 seconds and clipboard was
   never set.

2. Lazy clipboard data lost on exit: setData("image/png") puts a
   lazy reference on the macOS pasteboard — if the process exits before
   paste, data is lost ("Cannot keep promise"). The separate
   saveJpegToClipboardMacOS() workaround used osascript with a temp
   file and incorrectly declared JPEG data as PNG UTI («class PNGf»).

Fix

• Remove the isPrimaryInstance() check on macOS — use instance() to
  check if the daemon singleton exists in-process, matching non-macOS
  behavior. IPC fallback is preserved for true secondary processes (CLI).
• Add macOS branch in saveToClipboardMime() using setImageData(QImage)
  for native pasteboard persistence (public.tiff) plus setData() for
  exact format availability (PNG or JPEG).
• Delete saveJpegToClipboardMacOS() entirely — the fixed
  saveToClipboardMime() now handles both formats correctly.

All changes are guarded with #if defined(Q_OS_MACOS). The Linux, Windows,
and Wayland clipboard paths are unchanged.

Testing

Tested on macOS 26.4 (Tahoe), Apple Silicon, Qt 6.11.0:

• Tray menu Ctrl+C: clipboard set instantly (was 3s delay + empty)
• CLI flameshot full --clipboard: works via IPC fallback
• Both JPEG and PNG modes verified with `osascript -e 'clipboard info'

closes #4630

[borgmanJeremy]

This worked well and can be merged after addressing clang-format. The bug was introduced here. I am not sure why I didn't catch it in original testing: #4473

[Mitnitsky]

Addressed the clang-format, should pass now