wifi chip: fix off-center icon and drop dead CSS rule

wifi_img.set_hexpand(false) meant the icon packed at the start of the
32px icon-only chip with no leftover space for halign:Center to work
with, sitting visibly left of center on glass-workbench/liquid-motion.
hexpand(true) fixes the centering, but that alone silently bubbles the
expand flag up through connectivity_pair into the shared right-hand
stats box and the centerbox's end slot, blowing the wifi/vol cluster's
layout apart (confirmed via a --screenshot bar capture: the whole
cluster jumped left against the clock with a huge gap before
battery/hamburger). connectivity_pair.set_hexpand(false) pins the
box's own expand explicitly so the fix stays contained to this chip.

Also drops the dead .wifi-pair { padding: 6px } rule: it can never
apply since .stat-pair.icon-only's two-class selector always beats
its one-class specificity regardless of source order, so the intended
padding never actually rendered.

src/theme.rs

@@ -249,7 +249,6 @@ fn load_css() -> String {
              border-radius: 3px; min-height: 6px; }}\
          progressbar.osd-bar trough progress {{ background-image: none; background-color: @accent;\
              border-radius: 3px; min-height: 6px; }}\
-         .wifi-pair {{ padding: 6px; }}\
          window.breadbar-panel {{ background-color: alpha(@bg, 0.72); color: @on-bg;\
              border-radius: 14px; border: 1px solid alpha(@on-bg, 0.12); }}\
          window.breadbar-dismiss, .breadbar-dismiss-hit {{\