refactor(ui): settings-card primitives + fix threshold clamp / card misgroup (#161)

Tier-3 frontend DRY for the ML settings cards, plus the F-D2 clamp bug and the
F-D3 card misgrouping.

New primitives (components/common + composables):
- <SettingToggleRow> — the accent-icon + .fc-section-h label + right-aligned
  switch row (HeadsCard x3, CropProposersCard). iconColor prop absorbs the
  on/off dim.
- <SettingNumberField> — compact numeric field that CLAMPS to [min,max] on
  commit. This fixes F-D2: HeadsCard/CropProposersCard previously sent
  Number(raw) straight to the API, so an out-of-range threshold bounced off the
  400 validator (only TranslationCard clamped). density prop for the grid cards.
- useSettingSave(patchFn) — the busy + patch + toast + revert-on-failure flow
  each card hand-rolled (HeadsCard x6 handlers, CropProposersCard, MLBackfillCard,
  VideoEmbeddingCard). Returns ok/false for the optimistic-switch revert.

Adopted in HeadsCard, CropProposersCard, MLBackfillCard (handler only — its
plain labelled switch is a different affordance), VideoEmbeddingCard.

F-D3: MLThresholdSliders.vue actually rendered a "Video embedding" (frame-
sampling) card but sat under "Tagging → Suggestion thresholds". Renamed it
VideoEmbeddingCard.vue and moved it to the "GPU agent & embeddings" section.

Left deliberately (over-DRY guard): TranslationCard uses an inline error ALERT
(not a toast), already clamps its confidence with a NaN fallback, and lives on
the ImportStore — a genuinely different save pattern, so forcing it onto
useSettingSave would change its UX.

Behaviour-preserving refactor; CI has no Vue type-check so this needs a live
UI pass (toggles persist + revert on failure, thresholds clamp on blur, video
card now under Embeddings).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NsmJSQxnNxGgtM5Yz4GAqi

frontend/src/composables/useSettingSave.js

@@ -0,0 +1,33 @@
+import { ref } from 'vue'
+import { toast } from '../utils/toast.js'
+
+// The shared "persist a settings patch" flow for the ML settings cards. Flips a
+// busy flag, calls the store's patch (which rethrows on failure), toasts
+// success/error, and returns true/false so a toggle handler can revert its
+// optimistic switch on failure. Centralises the try/catch/toast the cards each
+// hand-rolled (HeadsCard x6, CropProposersCard, MLBackfillCard) — and where the
+// threshold-clamp drifted; the clamp now lives in <SettingNumberField>.
+//
+// Pass the store's patch fn, e.g. useSettingSave(ml.patchSettings).
+export function useSettingSave(patchFn) {
+  const busy = ref(false)
+
+  // opts.successMessage — toast on success (toggles announce their new state;
+  // silent field-saves omit it). opts.errorPrefix — the failure toast prefix
+  // ("Could not save" default; toggles used "Could not update").
+  async function save(patch, { successMessage = '', errorPrefix = 'Could not save' } = {}) {
+    busy.value = true
+    try {
+      await patchFn(patch)
+      if (successMessage) toast({ text: successMessage, type: 'success' })
+      return true
+    } catch (e) {
+      toast({ text: `${errorPrefix}: ${e.message}`, type: 'error' })
+      return false
+    } finally {
+      busy.value = false
+    }
+  }
+
+  return { busy, save }
+}