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

internal/ui/probe_other.go

@@ -4,5 +4,6 @@ package ui
 
 import "time"
 
-// probeKittyGraphics is not implemented here; the block characters are used.
-func probeKittyGraphics(time.Duration) bool { return false }
+// probeTerminal is not implemented here: block characters and the default
+// colors are used.
+func probeTerminal(time.Duration) terminalInfo { return terminalInfo{} }