feat: redesign Settings with left sidebar tabs per Apple HIG

Four tabs (General, Timer, Billing, Data) with icon + label sidebar,
amber active indicator, auto-save on change, progressive disclosure
for timer settings, and danger zone isolated within Data tab.

src/views/Settings.vue

@@ -1,12 +1,40 @@
 <template>
-  <div class="p-6 max-w-xl">
-    <h1 class="text-[1.5rem] font-medium text-text-primary mb-8">Settings</h1>
+  <div class="flex h-full">
+    <!-- Sidebar -->
+    <aside class="w-44 bg-bg-surface border-r border-border-subtle shrink-0 p-3">
+      <h1 class="text-[0.8125rem] font-medium text-text-tertiary uppercase tracking-[0.08em] px-2 mb-3">Settings</h1>
+      <nav class="flex flex-col gap-0.5">
+        <button
+          v-for="tab in tabs"
+          :key="tab.id"
+          @click="activeTab = tab.id"
+          class="relative flex items-center gap-2.5 px-2.5 py-2 rounded text-[0.8125rem] transition-colors duration-150"
+          :class="activeTab === tab.id
+            ? 'bg-bg-elevated text-text-primary'
+            : 'text-text-secondary hover:bg-bg-elevated hover:text-text-primary'"
+        >
+          <!-- Active indicator -->
+          <div
+            v-if="activeTab === tab.id"
+            class="absolute left-0 top-1.5 bottom-1.5 w-[2px] rounded-full bg-accent"
+          />
+          <component :is="tab.icon" class="w-4 h-4 shrink-0" :stroke-width="1.5" />
+          {{ tab.label }}
+        </button>
+      </nav>
+    </aside>
+
+    <!-- Content pane -->
+    <div class="flex-1 p-6 overflow-y-auto">
+      <!-- General -->
+      <div v-if="activeTab === 'general'">
+        <h2 class="text-[1.125rem] font-medium text-text-primary mb-6">General</h2>
 
-    <!-- 1. Appearance — most frequently adjusted -->
-    <section class="bg-bg-surface rounded-lg p-5 mb-4">
-      <h2 class="text-xs text-text-tertiary uppercase tracking-[0.08em] font-medium mb-4">Appearance</h2>
         <div class="flex items-center justify-between">
+          <div>
             <p class="text-[0.8125rem] text-text-primary">UI Scale</p>
+            <p class="text-[0.6875rem] text-text-tertiary mt-0.5">Adjust the interface zoom level</p>
+          </div>
           <div class="flex items-center gap-2">
             <button
               @click="decreaseZoom"
@@ -25,15 +53,19 @@
             </button>
           </div>
         </div>
-    </section>
+      </div>
 
-    <!-- 2. Timer — behavioral settings -->
-    <section class="bg-bg-surface rounded-lg p-5 mb-4">
-      <h2 class="text-xs text-text-tertiary uppercase tracking-[0.08em] font-medium mb-4">Timer</h2>
-      <div class="space-y-4">
+      <!-- Timer -->
+      <div v-if="activeTab === 'timer'">
+        <h2 class="text-[1.125rem] font-medium text-text-primary mb-6">Timer</h2>
+
+        <div class="space-y-5">
           <!-- Idle Detection toggle -->
           <div class="flex items-center justify-between">
+            <div>
               <p class="text-[0.8125rem] text-text-primary">Idle Detection</p>
+              <p class="text-[0.6875rem] text-text-tertiary mt-0.5">Detect when you're idle and prompt to continue or stop</p>
+            </div>
             <button
               @click="toggleIdleDetection"
               :class="[
@@ -50,7 +82,7 @@
             </button>
           </div>
 
-        <!-- Reminder Interval — indented child of idle detection, progressive disclosure -->
+          <!-- Reminder Interval — progressive disclosure -->
           <div
             v-if="idleDetection"
             class="flex items-center justify-between pl-4 border-l-2 border-border-subtle ml-1"
@@ -69,11 +101,12 @@
             />
           </div>
         </div>
-    </section>
+      </div>
+
+      <!-- Billing -->
+      <div v-if="activeTab === 'billing'">
+        <h2 class="text-[1.125rem] font-medium text-text-primary mb-6">Billing</h2>
 
-    <!-- 3. Billing — less frequently changed -->
-    <section class="bg-bg-surface rounded-lg p-5 mb-4">
-      <h2 class="text-xs text-text-tertiary uppercase tracking-[0.08em] font-medium mb-4">Billing</h2>
         <div class="flex items-center justify-between">
           <div>
             <p class="text-[0.8125rem] text-text-primary">Default Hourly Rate</p>
@@ -92,11 +125,14 @@
             />
           </div>
         </div>
-    </section>
+      </div>
 
-    <!-- 4. Data — export and management -->
-    <section class="bg-bg-surface rounded-lg p-5 mb-4">
-      <h2 class="text-xs text-text-tertiary uppercase tracking-[0.08em] font-medium mb-4">Data</h2>
+      <!-- Data -->
+      <div v-if="activeTab === 'data'">
+        <h2 class="text-[1.125rem] font-medium text-text-primary mb-6">Data</h2>
+
+        <div class="space-y-6">
+          <!-- Export -->
           <div class="flex items-center justify-between">
             <div>
               <p class="text-[0.8125rem] text-text-primary">Export All Data</p>
@@ -109,11 +145,10 @@
               Export
             </button>
           </div>
-    </section>
 
-    <!-- 5. Danger Zone — destructive actions, isolated at bottom -->
-    <section class="mt-8 rounded-lg border border-status-error/20 p-5">
-      <h2 class="text-xs text-status-error uppercase tracking-[0.08em] font-medium mb-4">Danger Zone</h2>
+          <!-- Danger Zone -->
+          <div class="mt-8 rounded-lg border border-status-error/20 p-5">
+            <h3 class="text-xs text-status-error uppercase tracking-[0.08em] font-medium mb-4">Danger Zone</h3>
             <div class="flex items-center justify-between">
               <div>
                 <p class="text-[0.8125rem] text-text-primary">Clear All Data</p>
@@ -126,7 +161,10 @@
                 Clear Data
               </button>
             </div>
-    </section>
+          </div>
+        </div>
+      </div>
+    </div>
 
     <!-- Clear Data Confirmation Dialog -->
     <div
@@ -162,15 +200,25 @@
 </template>
 
 <script setup lang="ts">
-import { ref, onMounted } from 'vue'
+import { ref, onMounted, markRaw } from 'vue'
 import { invoke } from '@tauri-apps/api/core'
-import { Plus, Minus } from 'lucide-vue-next'
+import { Settings as SettingsIcon, Clock, Receipt, Database, Plus, Minus } from 'lucide-vue-next'
 import { useSettingsStore } from '../stores/settings'
 import { useToastStore } from '../stores/toast'
 
 const settingsStore = useSettingsStore()
 const toastStore = useToastStore()
 
+// Tabs
+const tabs = [
+  { id: 'general', label: 'General', icon: markRaw(SettingsIcon) },
+  { id: 'timer', label: 'Timer', icon: markRaw(Clock) },
+  { id: 'billing', label: 'Billing', icon: markRaw(Receipt) },
+  { id: 'data', label: 'Data', icon: markRaw(Database) },
+]
+
+const activeTab = ref('general')
+
 // Settings state
 const hourlyRate = ref(0)
 const idleDetection = ref(true)