Add standard CSD titlebutton icons + fix mac hover icons (GTK3 + GTK4)

[newman2x]

Problem

The standard (non-macos) variants of Orchis had no styling for CSD titlebuttons.
Without --tweaks macos, window buttons rendered using GTK's default fallback —
no themed icons, no hover or active states.

Under --tweaks macos, the %firefox_window_button hover icons had two issues:
SVG references pointed at the standard symbolic icons (not the mac-specific ones),
and the selector required .default-decoration which plain GTK3 CSD apps like
Celluloid never have, so hover icons were silently skipped on those apps.

---

From top to bottom:
GTK4
GTK3
XFWM4

Changes

src/_sass/gtk/_common-3.20.scss

• Add %standard_window_button placeholder: background-image SVG icons sized and
  centered on the button node, image { opacity: 0 } to hide GTK's native icon child,
  hover/active background using if($variant == 'light', rgba(black, 0.15), rgba(white, 0.15)),
  animation: none on active to prevent GTK ripple from expanding background-size
• Extend %standard_window_button via new @else branch in button.titlebutton block
  when $mac_style != 'true'
• Add @else branch for standard unmaximize icon when window is maximized
• Increase titlebar side padding from $space-size to $space-size + 4px
• Fix %mac_window_button margin: $space-size - 2px → $space-size
• Fix %firefox_window_button:
  • Move SVG references to base level (not inside &:hover, &:active)
  • Change to mac-specific SVGs (*-symbolic-mac.svg)
  • background-size: 0 0 at rest, revealed to 16px 16px on hover/active, so the icon
    is invisible until hovered without requiring background-image to be unset
  • Add image { opacity: 0 } to suppress native GTK icon
  • Broaden selector from .titlebar.default-decoration to .titlebar — plain GTK3
    CSD apps (e.g. Celluloid) never have .default-decoration, so they were silently
    excluded from hover icons under --tweaks macos
  • Fix maximized unmaximize selector: remove .default-decoration for same reason,
    update SVG reference to unmaximize-symbolic-mac.svg

src/_sass/gtk/_common-4.0.scss

• Add %standard_window_button placeholder with same approach as GTK3, using
  > image (direct child selector, required in GTK4) with opacity: 0 to hide the
  native icon
• Extend %standard_window_button via @else branch, replacing the previous bare
  image reset block
• Fix headerbar border-radius corner-cropping when maximized/tiled/fullscreen by
  adding a .maximized & override block
• Fix headerbar windowcontrols border-spacing: $space-size → 2px
• Fix %mac_window_button margins: $space-size - 2px → $space-size
• Add mac hover icon overlay on > image directly — required because %mac_window_button
  draws colored circles via > image { background-color }, so background-image on the
  button node renders behind the child and is invisible
• Add color: transparent on mac > image to suppress native GTK symbolic icon
  (rendered via currentColor) without affecting background-color or background-image
• Add @if/@else unmaximize block targeting > image on hover/active

src/gtk/scalable/

• Add *-symbolic-dark.svg / @2.svg copies with fill:#ffffff for dark variant,
  used automatically via $asset-suffix interpolation
• Add *-symbolic-mac.svg / @2.svg files with fill:#ffffff for mac hover icons
  (used by %firefox_window_button GTK3 and mac hover blocks GTK4)

---

Notes

• %mac_window_button colored circles are completely unchanged
• Tested on Linux Mint 22.3 XFCE with GTK3 (Celluloid, gnome-calculator) and
  GTK4/libadwaita (gnome-calendar, baobab)
• -gtk-recolor(url(...)) does not work reliably on this GTK3 version for non-black
  fills — separate SVG copies are used instead
• GTK Inspector shows /* FIXME */ next to -gtk-scaled() in GTK4 — display bug
  in the Inspector only, SVG loads correctly at runtime
• core.sh is not modified — it already copies src/gtk/scalable/ to both
  gtk-3.0/assets/ and gtk-4.0/assets/ on every install