feat(library): set the category on many selected prompts (#437) (#446)

The Prompt library shelf gets the concept's Images selection checkbox on each
card and its .bulkbar (N selected, Set category, Clear), recorded as D148.
Set category opens the shared one-value dialog and posts
{prompt_ids, category, expected_epoch} to POST /prompt_manager/bulk/category:
the server checks the storage epoch the shelf was read under, then sets the
category on the Library parents in one BEGIN IMMEDIATE transaction, writing
only rows that change. A restored storage answers 409 storage-changed and
writes nothing. The legacy admin and images_filter bodies are unchanged.

This replaces the legacy admin page's bulk Set Category, a cutover
prerequisite (#334).

Closes #437

web/js/workspace/workspace.css

@@ -264,6 +264,9 @@ main:focus-visible { outline: 2px solid var(--ca-focus); outline-offset: 4px; }
 /* #249 D23: a parent with one preview, or none, fills the art row instead of leaving the concept's second cell empty. */
 .prompt-card .card-art > :only-child { grid-column: 1 / -1; /* D23 */ }
 .prompt-card .card-art > .small { display: grid; place-content: center; min-width: 0; color: var(--ca-muted); /* D23 */ }
+/* #437 D148: the card's selection checkbox is the concept's Images `.image-overlays`, placed over the art as on an image
+   card; `.media-art` carries the concept's `position: relative` there, the prompt card carries it here. */
+.prompt-card { position: relative; /* D148 */ }
 
 /* Prompt library shelf (#249 PR E). #249 D3: rows stay windowed, as the Images grid's do; the concept's `.library-grid`
    tracks and breakpoints lay them out, each row spans them, and appended rows never anchor the scroll. The row's