feat(desktop): persistent terminal pane + fullscreen takeover

Adds a VSCode-style "focus terminal" toggle to the right sidebar's Terminal
tab that takes over the chat pane area without unmounting the shell. The
xterm host is mounted once at the layout root and CSS-overlayed onto
whichever <TerminalSlot /> is currently active, so the PTY session,
scrollback, selection, focus, and WebGL renderer survive every toggle.

Also:
- WebGL renderer (matching dashboard ChatPage) so Hermes' TUI skins paint
  faithfully instead of muting through xterm's default DOM renderer
- File drag/drop from the project tree or OS into xterm — paths are
  shell-quoted (zsh/bash/pwsh/cmd) and written straight into the PTY
- Solarized dark canvas with brights promoted to real accent variants
  (Schoonover's UI-gray brights washed out every TUI accent)
- Strip NO_COLOR/FORCE_COLOR/COLORFGBG/TERM=dumb leaking from non-tty
  parents (CI runners, Cursor's agent shell) so the embedded shell gets
  truecolor regardless of how Electron was launched
- rAF-debounced ResizeObserver — running fit.fit() synchronously during
  sibling pane transitions crashed the WebGL texture-atlas rebuild

apps/desktop/electron/main.cjs

@@ -3096,7 +3096,15 @@ function terminalShellEnv() {
     }
   }
 
-  env.COLORTERM = env.COLORTERM || 'truecolor'
+  // Strip color/theme-detection vars that ride along when Electron is launched
+  // from a non-tty agent shell (Cursor's runner sets NO_COLOR/FORCE_COLOR=0
+  // /TERM=dumb; some terminals set COLORFGBG which would flip Hermes' TUI into
+  // light-mode). Our PTY is a real xterm-compat terminal — force truecolor.
+  delete env.NO_COLOR
+  delete env.FORCE_COLOR
+  delete env.COLORFGBG
+
+  env.COLORTERM = 'truecolor'
   env.LC_CTYPE = env.LC_CTYPE || 'UTF-8'
   env.TERM = 'xterm-256color'
   env.TERM_PROGRAM = 'Hermes'