Use the terminal's own background color

The panes used palette color 0, which color themes such as Catppuccin set
to a grey lighter than their background: everything looked washed out and
grey text almost disappeared (seen in Ghostty on macOS). The startup probe
now also asks for the background color (OSC 11), and with 24-bit color and
a dark background that color is used. The probe always runs at startup, so
a later switch to real images in the Config view never has to ask the
terminal while the screen owns it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- **The terminal's own background:** the panes used color 0 of the terminal's palette ("black"),
+  which many color themes set to a grey lighter than their background (e.g. Catppuccin), so the
+  whole program looked washed out and grey text almost disappeared. The terminal is now asked for
+  its background color at startup and that is used (with 24-bit color and a dark background).
 - **Smooth scrolling on huge terminals:** while an arrow key is held, the poster waits until the
   selection rests (120 ms). Every poster rewrites all of its cells, which on a terminal zoomed far
   out was more output per step than the terminal could draw, so moving through a list lagged.