fix(labels): replace incorrect ams_30x15 preset with correct AMS holder sizes (#1426)

Reporter — the same person who originally requested the labels
  feature in #809 — discovered that the ams_30x15 preset's 30x15 mm
  dimension didn't actually fit any variant of the MakerWorld AMS
  Filament Label Holder (model 752566) it advertised. Two new
  presets replace it:

  - ams_holder_74x33 (74 x 33 mm) matches the printable label STL
    bundled in the MakerWorld project
  - ams_holder_75x55 (75 x 55 mm) fits the cardstock-insert variant
    the reporter validated on bench

  Both cross the 20 mm height threshold so they land in the roomy
  layout branch — swatch on the left, QR on the right, multi-line
  text (brand, material, hex code, spool ID) in the middle. The
  old 30x15 mm preset couldn't fit a QR code; the new ones do.

  No DB migration: the preset name was never persisted. Callers
  scripting the old ams_30x15 value get a clean 422 at the route's
  Literal validator with the new valid values listed.

  i18n: replaced inventory.labels.templates.ams.{label,hint} with
  amsHolderSmall and amsHolderLarge across all 8 locales with real
  translations; parity guard cleaned of the stale English-fallback
  cognate entries. Parity holds at 4856 leaves per locale.

  Tests: backend label renderer + integration tests cover both new
  presets; LabelTemplatePickerModal test updated for the 6-button
  grid and the new template value in the API-call assertion.

frontend/src/components/LabelTemplatePickerModal.tsx

@@ -33,10 +33,16 @@ interface TemplateOption {
 
 const TEMPLATE_OPTIONS: TemplateOption[] = [
   {
-    value: 'ams_30x15',
-    i18nKey: 'ams',
-    fallbackLabel: 'AMS holder (30 × 15 mm)',
-    fallbackHint: 'Single label per page; fits the popular AMS filament label holder.',
+    value: 'ams_holder_74x33',
+    i18nKey: 'amsHolderSmall',
+    fallbackLabel: 'AMS holder — small (74 × 33 mm)',
+    fallbackHint: 'Single label per page; matches the printable label from MakerWorld model 752566 (AMS Filament Label Holder).',
+  },
+  {
+    value: 'ams_holder_75x55',
+    i18nKey: 'amsHolderLarge',
+    fallbackLabel: 'AMS holder — large (75 × 55 mm)',
+    fallbackHint: 'Single label per page; fits the cardstock-insert variant of the AMS Filament Label Holder. Roomy enough for swatch, brand, material, ID, and QR code.',
   },
   {
     value: 'box_40x30',