Add --help, terminal scrollback + scrollbar, run-dialog fix, client-safe exit, true rounded corners

- -h/--help prints a full usage overview (modes, config keys, keys, mouse)
- Run dialog now executes the typed command directly in the new window
  (previously it spawned the default shell and typed the command into
  it, which showed up as just an empty terminal)
- Virtual terminals get vt100 scrollback (config scrollback_lines,
  default 1000): mouse wheel scrolls history when the app doesn't
  capture the mouse, arrow-key fallback on the alternate screen, any
  keypress jumps back to the bottom; optional scrollbar on the right
  border (show_scrollbar) plus configurable default_window_size
- Exit triggered from a connected display client (ctrl+q / panel
  button) now only disconnects that client — the daemon/standalone
  session and its programs keep running
- Windows now paint the border zone in the desktop background color so
  rounded corners actually look rounded instead of a filled square
  with an inner rounded frame; popup dialogs rounded too
- Integration suite extended to 19 checks (scrollback, run dialog,
  client-safe exit) — all passing

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

config.toml

@@ -11,6 +11,17 @@ background_image = "/home/brasse/Bilder/backrunder/pexels-veeterzy-38136.jpg"
 # Standard-shell för nya terminaler. Om inte satt → $SHELL eller /bin/bash.
 # default_shell = "/bin/fish"
 
+# Antal raders scrollback-historik per virtuell terminal (default: 1000).
+# Scrolla med mushjulet i fönster vars app inte själv fångar musen;
+# tangenttryck hoppar tillbaka till botten.
+# scrollback_lines = 1000
+
+# Visa scrollbar på terminalfönstrens högerkant när historik finns (default: true)
+# show_scrollbar = true
+
+# Standardstorlek [bredd, höjd] för nya terminalfönster (default: [82, 26])
+# default_window_size = [82, 26]
+
 [[panel]]
 position = "top"
 file = "panels/topbar.toml"