refactor: extract reusable UI components and composables from TextSettings

Major refactoring to improve code quality and reduce duplication:

TextSettings.vue: 1127 → 269 lines (-76%)

New composables:
- useCssUpdater.js: generic CSS update/remove functions
- useCssSync.js: CSS parsing to form fields

New UI components:
- UnitToggle.vue: reusable unit selector buttons
- InputWithUnit.vue: number input with unit toggle
- MarginEditor.vue: simple/detailed margin editor with sync

Benefits:
- Reusable components for other settings panels
- Centralized CSS manipulation logic
- Better separation of concerns
- Easier to test and maintain

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/components/ui/UnitToggle.vue

@@ -0,0 +1,28 @@
+<template>
+  <div class="unit-toggle">
+    <button
+      v-for="unit in units"
+      :key="unit"
+      type="button"
+      :class="{ active: modelValue === unit }"
+      @click="$emit('update:modelValue', unit)"
+    >
+      {{ unit }}
+    </button>
+  </div>
+</template>
+
+<script setup>
+defineProps({
+  modelValue: {
+    type: String,
+    required: true
+  },
+  units: {
+    type: Array,
+    default: () => ['mm', 'px']
+  }
+});
+
+defineEmits(['update:modelValue']);
+</script>