refactor: simplify mobile navigation

frontend/src/App.vue

@@ -479,8 +479,8 @@ onMounted(bootstrap)
       <div v-else class="paper"><ListChecks/><b>选中一个任务</b><p>日期、优先级、子任务和 Markdown 备注会出现在这里。</p></div>
     </aside>
 
-    <div v-if="mobileMore" class="more-mask" @click.self="mobileMore=false"><section id="mobile-more-menu" class="more-sheet" role="dialog" aria-modal="true" aria-label="更多导航"><div class="more-sheet-head"><b>更多</b><button class="icon" aria-label="关闭更多菜单" @click="mobileMore=false"><X/></button></div><button @click="switchView('upcoming')"><CalendarDays/>最近 7 天</button><button @click="switchView('trash')"><Trash2/>回收站</button><button @click="switchView('settings')"><Settings/>设置与数据</button><button @click="mobileMore=false;mobileSidebar=true"><Menu/>全部清单</button></section></div>
-    <nav class="bottom" aria-label="主要导航"><button :class="{active:activeView==='today'}" @click="switchView('today')"><ListTodo/><span>今天</span></button><button :class="{active:activeView==='tasks'}" @click="switchView('tasks',lists.find(l=>l.is_inbox)?.id)"><Inbox/><span>任务</span></button><button :class="{active:activeView==='habits'}" @click="switchView('habits')"><Repeat2/><span>习惯</span></button><button :class="{active:mobileMore||['upcoming','trash','settings'].includes(activeView)}" aria-haspopup="dialog" aria-controls="mobile-more-menu" :aria-expanded="mobileMore" @click="mobileMore=!mobileMore"><Ellipsis/><span>更多</span></button></nav>
+    <div v-if="mobileMore" class="more-mask" @click.self="mobileMore=false"><section id="mobile-more-menu" class="more-sheet" role="dialog" aria-modal="true" aria-label="更多导航"><div class="more-sheet-head"><b>更多</b><button class="icon" aria-label="关闭更多菜单" @click="mobileMore=false"><X/></button></div><button @click="switchView('settings')"><Settings/>设置与数据</button></section></div>
+    <nav class="bottom three-items" aria-label="主要导航"><button :class="{active:activeView==='today'}" @click="switchView('today')"><ListTodo/><span>今天</span></button><button :class="{active:activeView==='habits'}" @click="switchView('habits')"><Repeat2/><span>习惯</span></button><button :class="{active:mobileMore||['tasks','upcoming','trash','settings'].includes(activeView)}" aria-haspopup="dialog" aria-controls="mobile-more-menu" :aria-expanded="mobileMore" @click="mobileMore=!mobileMore"><Ellipsis/><span>更多</span></button></nav>
     <button v-if="['tasks','today','upcoming'].includes(activeView)" class="fab" aria-label="添加任务" @click="focusQuick"><CirclePlus/></button>
     <Transition name="toast"><div v-if="notice" class="toast" role="status">{{notice}}</div></Transition>
     <div v-if="error" class="error-toast" role="alert">{{error}}<button @click="error=''"><X/></button></div>