printedparts stage 12: admin settings page + settings-rail card

PrintedPartsSettings edits the four plugin settings (code prefix,
default threshold, kiosk badge policy, alert recipients) through the
core settings API; the route rides the plugin's router file and the
settings shell nests it into the rail; get_settings_cards contributes
the catalog card while the plugin is enabled.

docs/PLUGIN-LAB-PRINTEDPARTS.md

@@ -306,6 +306,20 @@ CONTRACT ADDITION, because the mailer was not on the plugin surface:
    crossing, no re-fire while below, rearm after restock (see
    `test_lowstock_alert_fires_on_crossing_only`).
 
+## Stage 12 (extension) - the admin settings page
+
+A get_settings_cards card needs a PAGE to link, which is why stage 9 deferred
+it. The page is ordinary:
+1. `frontend/src/views/settings/PrintedPartsSettings.vue` - load the four
+   keys via `settingsApi.list({category: 'printedparts'})`, save each with
+   `settingsApi.update(key, value)` (admin-gated server-side).
+2. Route in the PLUGIN's router file with path `settings/printedparts` +
+   `requiresAuth, requiresAdmin, plugin` meta - the router shell
+   automatically nests any `settings/...` path under the two-pane settings
+   rail.
+3. `get_settings_cards` on the plugin pointing at `/settings/printedparts` -
+   the card appears in the rail's catalog while the plugin is enabled.
+
 ---
 
 ## Where each pattern lives (cheat sheet)