feat(keybinds): capture region/screen to file

Add SHIFT+Print (region → file) and CTRL+Print (screen → file) next to
the existing bare Print (region → clipboard). Each saves a timestamped
PNG under ~/Pictures/Screenshots and toasts the path — the same grim/slurp
plumbing and directory the Capture menu's "→ file" rows already use.
Single-source keybinds.nix, so the SUPER+? cheatsheet lists both new rows.

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

agent/HARDWARE-QUEUE.md

@@ -236,6 +236,11 @@ QA machine), the **T14s** (webcam case).
       unchanged (the bar still reserves its space). Accepted trade-off
       of `layer: bottom`: a floating window dragged over the top strip
       can now overlap the bar — confirm that's the only regression.
+- [ ] **Capture-to-file keybinds** (iteration #59, item 38) — after
+      `home-update` + relogin: SHIFT+Print prompts a region select then
+      saves a PNG under ~/Pictures/Screenshots (toast shows the path);
+      CTRL+Print saves the whole screen the same way; bare Print still
+      copies a region to the clipboard. Both new rows appear in SUPER+?.
 - [ ] **rofi menu polish** (iteration #58, item 39 + #56/#57 34/40) —
       after `home-update`: (a) menu › System › Power profile shows a
       colored icon per profile (performance/balanced/power-saver), not