feat: scale the whole session, not just the classroom's own windows

UI_SCALE only ever reached the surfaces we draw ourselves. Chrome stayed
tiny — its tabs, omnibox and menus are Chrome's own pixels, and Ctrl+
only scales page content, which is no use to a volunteer reading a
toolbar from across the room.

UI_SCALE is a property of the television, not of one program, so it now
sets sway's output scale, which every client is measured against.
classroom-scale runs first in the session and applies it. The display
mode is untouched: the panel stays at 1920x1080 and only the logical
coordinate space shrinks, which is what makes everything in it larger.

Our own surfaces stop scaling themselves. They are measured in the same
logical pixels sway is already scaling, so applying UI_SCALE again would
double it. SHELL_SCALE is that decision in one named place, and setting
it back to ui_scale(config) would restore per-surface scaling without
touching a template.

Measured at 1920x1080 with Chrome 152. Chrome's tab strip 40 -> 49 ->
59px against 1.0, 1.25, 1.5; sway's title bar 25 -> 31 -> 38px; the
classroom dock 74 -> 92 -> 110px. Everything scales once, together, and
1.0 is identical to no scaling at all.

Two tradeoffs, both measured rather than assumed. GTK 3 has no
fractional scaling, so at 1.5 it renders at 2x and sway scales down —
very slightly softer, invisible at classroom distance. XWayland is
softer too: terminal text at 1.5 showed about 50% more intermediate-tone
pixels than at 1.0. VLC is the only XWayland application here, and in
DVD mode none of its interface is on screen, only video from a 720x480
disc that is being upscaled regardless. Chrome is a native Wayland
client and stays sharp, which is what matters for reading.

Co-Authored-By: Claude Opus 5 <[email protected]>

lib/classroom_config.py

@@ -60,6 +60,20 @@ def ui_scale(config):
     return min(max(value, UI_SCALE_MIN), UI_SCALE_MAX)
 
 
+# The scale the classroom's own GTK surfaces render at.
+#
+# UI_SCALE is applied by sway to the whole output (see classroom-scale),
+# which scales Chrome, VLC and every other application along with the
+# dock. Our surfaces are measured in the same logical pixels sway is
+# already scaling, so scaling them here as well would apply UI_SCALE
+# twice and the dock would come out half again too big.
+#
+# This is the seam: if compositor scaling ever has to be abandoned,
+# changing this back to ui_scale(config) restores per-surface scaling
+# without touching the templates.
+SHELL_SCALE = 1.0
+
+
 def scaled(value, scale):
-    """A pixel measurement at the configured scale, never smaller than 1."""
+    """A pixel measurement at the given scale, never smaller than 1."""
     return max(1, int(round(value * scale)))