fix(rofi): Display menu Back returns to System on single-monitor

The mode picker's Back did `exec "$0" display`. On a single-monitor
machine with no display profiles the output chooser is skipped, so
re-running `display` dropped straight back into the same mode picker — an
apparent no-op. Track modeBack (display when the chooser was shown, else
system) and dispatch Back to it, so single-monitor Back leaves Display.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -50,12 +50,6 @@ still doesn't open the keybindings cheatsheet. Re-diagnose: confirm the
 Hyprland bind's modmask/keysym against the *actual* layout, whether rofi
 2.0.0 changed anything, and that the cheatsheet path still resolves.
 
-### 40. Display rofi menu "Back" doesn't return `[bug]`
-(Raised by Bernardo, 2026-07-07 — item 10.) In System › Display, the
-Back row fails to return to the previous menu (other submenus' Back work
-— cf. the item 24 audit). Trace the Display handler's Back case /
-dispatch target.
-
 ### 39. Icons in the power-profile rofi menu
 (Raised by Bernardo, 2026-07-07 — item 9.) The power-profile picker
 (rofi.nix `power-profile`) lists profiles as bare text; add per-profile