Docker: delete volumes nothing uses, prune anonymous ones; images marked in use, remove unused

A named volume is deleted by typing its name, one at a time; the API wants it named
twice; nothing is forced. Prune touches anonymous volumes only.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

PLAN.md

@@ -239,7 +239,13 @@ something long in one, switch away and back, refresh — it's all still there.
   not erased; volumes and bind-mounted data are never touched. A stopped container can be
   **removed** (a running one must be stopped first, on purpose). Images: **pull** (a followed
   task), **remove** (unforced: Docker refuses one still in use) and **prune** (dangling only).
-  Still absent on purpose: removing volumes and networks, and anything forced.
+  **Volumes (2026-09-20):** each says which containers use it; one that nothing uses can be
+  deleted — a named one by typing its name (it may be the data of a stack that is only
+  down), an anonymous one by confirming its full name — and "Prune anonymous" clears the
+  nameless leftovers of recreated containers, never named volumes. Images are marked "in
+  use", and "Remove unused" takes every image no container uses. Nothing is ever forced:
+  Docker refuses what is still in use, and that refusal is the answer. Still absent:
+  removing networks.
 - **Skins** — 13 since 2026-09-19 (`themes.ts`: a palette is one `mk(…)` line): Dark,
   Light, Cyberpunk, Retro, **HUD** (hairlines of cyan on near-black, corner ticks, thin
   type), Nord, Gruvbox, Catppuccin + Latte, Dracula, Tokyo Night, Solarized Dark + Light;