fix(workspace): keep open pages valid across a restart and recover a changed source (#430)

The gallery root scope and the Images listing cursor were keyed with a
per-process random key, so every ComfyUI restart made an open tab's
scopes answer 409 source-changed (blank Tag image preview, a Generate
that did nothing) and its cursors fail Load more.

- utils/workspace_keys.py: one random secret in workspace-scope.key
  beside the database (owner-only, never config.json or the database),
  two derived keys; per-process fallback with a warning. Move library
  copies it.
- Cursor v2 binds the listing scope in its MAC instead of sending an
  unkeyed hash of the root paths; an unverifiable cursor answers 400
  cursor-stale and Images offers Refresh instead of a dead Retry.
- POST /prompt_manager/images/rescope maps a stale target to its
  current root only when the held scope was issued for it.
- Images finds a renumbered file again from the inspector, the viewer
  and the Tag image dialog (the dialog stays open, draft kept); nothing
  else is kept by an old root_index:path key.
- Tag image dialog: a state that blocks Save shows its reason above the
  actions with Refresh, disabled actions look disabled, and an empty
  preview says "Preview unavailable" (D147).

web/js/workspace/workspace.css

@@ -450,3 +450,9 @@ pre.prompt-text { font: inherit; font-size: 12px; line-height: 1.75; margin-bloc
 /* #332 D146: the category chip names its origin (" · this image", " · prompt", " · recipe") in muted text that never
    splits; at 390px the chip may wrap between the category and its origin. */
 .chip .chip-source { color: var(--ca-muted); white-space: nowrap; }
+/* D147 (#430): a state that blocks Save shows its reason on its own row above the sticky actions, with Refresh for a
+   changed source; the disabled actions look disabled; a refused capture's empty preview says so. */
+.tagging-modal .dialog-actions [aria-disabled="true"], .tagging-modal .tag-model-panel [aria-disabled="true"] { opacity: .55; cursor: not-allowed; /* D147 */ }
+.tagging-modal .tag-action-notice { flex: 1 0 100%; display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: 8px; margin: 0; color: var(--ca-text); font-size: 12px; /* D147 */ }
+.tagging-modal .tag-action-notice > span { flex: 1 1 16em; }
+.tag-image-preview.tag-preview-empty { display: grid; place-items: center; padding: 8px; text-align: center; font-size: 12px; color: var(--ca-muted); background: var(--ca-field); box-sizing: border-box; /* D147 */ }