Add h keybinding to toggle hold/release via postsuper -h/-H

Preserves the ! suffix from postqueue -p output to track hold status
in QueueEntry, and optimistically toggles it in the UI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -60,6 +60,7 @@ Subjects are fetched lazily in parallel via `postcat` as the list loads; a progr
 | `↑↓` / `Space` / `PgUp` / `PgDn` | Scroll      |
 | `H`             | Toggle full headers / short headers |
 | `s`             | Save raw EML to `~/QUEUEID.eml` |
+| `h`             | Toggle hold (`postsuper -h`/`-H`) |
 | `D`             | Delete message (`postsuper -d`) |
 | `F`             | Requeue message (`postsuper -r`) |
 | `v`             | Browse MIME parts               |