Fix macOS 26+ dock icon losing system theming (#8816)

[d3ns0n]

Problem

On macOS 26 (Tahoe) the Bambu Studio dock icon does not respect the system's
new icon theming (Liquid Glass / Tinted / Clear / Dark). When the app launches
it loses the themed appearance and shows a plain raster icon instead.

Root cause: on startup MainFrame sets a custom dock icon at runtime via a
wxTBI_DOCK taskbar icon (BambuStudio-mac_256px.ico). This calls
-[NSApp setApplicationIconImage:] under the hood, which overrides the system
theming that macOS applies to the app bundle icon (Icon.icns).

Fixes #8816

Change

Skip the runtime dock-icon override on macOS 26 and later, so the system-themed
bundle icon is used instead. Behaviour on macOS < 26 is unchanged.

• Add Slic3r::is_mac_os_at_least(int major) in MacUtils.mm, implemented with
  -[NSProcessInfo isOperatingSystemAtLeastVersion:]. This is a lower-bound
  check, so future releases (27+) are covered automatically, and it works
  regardless of the SDK the project is built with.
• Guard the #ifdef __APPLE__ dock-icon initialization in MainFrame with
  if (!is_mac_os_at_least(26)).

Testing

• Built and ran locally on macOS 26.5 (Apple Silicon, arm64).
• With the fix, the dock icon now follows the system Icon & Widget Style
  (verified with the "Tinted" style — the running app's dock icon stays tinted
  instead of reverting to the raster icon).
• Change is fully contained in #ifdef __APPLE__, so non-Apple builds are
  unaffected.

[d3ns0n]

Screenshots (app running on macOS 26.5, Apple Silicon)

Before → After — Icon & Widget Style: Tinted – Light

Before (current behaviour)	After (this PR)

With the fix the dock icon follows the system Icon & Widget Style instead of the raster override:

Dark	Tinted – Dark	Tinted – Light