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/components/common/SettingToggleRow.vue

@@ -0,0 +1,42 @@
+<!--
+  Canonical settings toggle row (DRY pass #161): an accent icon + an uppercase
+  .fc-section-h label + a right-aligned switch. Hand-rolled identically in the
+  ML settings cards (HeadsCard x3, CropProposersCard, MLBackfillCard).
+
+  Two-way binds `modelValue` (so the parent switch state stays optimistic) AND
+  emits `change` with the new boolean, so the parent can persist + revert on
+  failure — matching the prior `v-model` + `@update:model-value=handler` pattern.
+-->
+<template>
+  <div class="d-flex align-center mb-1" style="gap: 10px;">
+    <v-icon v-if="icon" size="18" :color="iconColor">{{ icon }}</v-icon>
+    <span class="fc-section-h">{{ label }}</span>
+    <v-switch
+      :model-value="modelValue"
+      :loading="loading"
+      :disabled="disabled"
+      hide-details density="compact" color="success" class="ml-auto"
+      @update:model-value="onSwitch"
+    />
+  </div>
+</template>
+
+<script setup>
+defineProps({
+  modelValue: { type: Boolean, default: false },
+  label: { type: String, default: '' },
+  icon: { type: String, default: '' },
+  // Icon tint. Default accent; pass null for the theme default (e.g. when a row
+  // is off). null (not undefined) so the default doesn't override it.
+  iconColor: { type: String, default: 'accent' },
+  loading: { type: Boolean, default: false },
+  disabled: { type: Boolean, default: false },
+})
+const emit = defineEmits(['update:modelValue', 'change'])
+
+function onSwitch(v) {
+  const b = !!v
+  emit('update:modelValue', b)
+  emit('change', b)
+}
+</script>