feat(menu): dimmed keybind hints on menu rows (item 28c)

The audit flagged the root picker as six short rows in ~760px of
empty width; the space now carries discoverability. menuHint renders
a dimmed pango span (size small, 55% alpha) from the SAME keybinds.nix
the Hyprland binds and the SUPER+? cheatsheet come from (prettyKeys),
so hints can never drift. Applied to the root picker (Apps special-
cased — it's rofi drun), all Tools leaves, and the System rows with
direct binds; those menus pass -markup-rows. Contains-glob matching
and the exact "↩ Back" row are unaffected.

Verified: V0; V1 (rendered menu bash -n, spans + 3x -markup-rows
present); visual — tokyo-night 1080p capture shows the hints dimmed
and subordinate on normal and selected rows alike.

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

agent/BACKLOG.md

@@ -69,10 +69,10 @@ further palette work graduates from slices (c)/(d) findings;
 headlessly at 1080p and screenshots desktop + open root menu (proven
 on tokyo-night); per-surface punch items from the first full-res
 review:
-  - **rofi root menu width:** ~760px for six short left-hugging rows —
-    either tighten, or use the space: right-aligned `SUPER+CTRL+<key>`
-    hint per row (keybinds.nix already knows them; discoverability
-    win).
+  - ✓ **rofi keybind hints** (shipped iteration #37): dimmed pango
+    hints on root/Tools/System rows, generated from keybinds.nix via
+    the cheatsheet's prettyKeys (single source, can't drift) —
+    capture-verified on tokyo-night.
   - **Icon weight mix `[human]`:** full-color Papirus icons in the
     menus vs all-monochrome bar glyphs — pick: symbolic/monochrome
     menu icons for one system, or color as the menu's identity.