feat(题目): 手机上「运行例子 / 提交」固定在屏幕底部，看着题目也能直接交

- 两个 44px 高的大按钮固定在底部，三个页签下都在；原来它们在「代码」页签顶上，
  和语言、更多挤成两行，看着题面想交一次还得先切回代码页签
- 「代码」页签的编辑器一直挂着（原来切走就卸载）：没打开过代码页签也能交，撤销历史也不丢
- 编辑器高度让出底部那条，最后几行不再被按钮盖住

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/oj/problem/components/MobileActionBar.vue

@@ -0,0 +1,70 @@
+<script setup lang="ts">
+import { useThemeVars } from "naive-ui"
+import { storeToRefs } from "pinia"
+import { useCodeStore } from "oj/store/code"
+import { useProblemStore } from "oj/store/problem"
+import { useSubmissionStore } from "oj/store/submission"
+import SubmitCode from "./SubmitCode.vue"
+
+/**
+ * 手机上屏幕底部固定的「运行例子 / 提交」（设计文档 5.6）。原来这两个按钮在「代码」页签
+ * 顶上，和语言、更多挤成两行；看着题面想交一次，还得先切回代码页签。
+ * 提交状态、代码都在 store 里，编辑器没挂着也能交。
+ */
+const theme = useThemeVars()
+const codeStore = useCodeStore()
+const submissionStore = useSubmissionStore()
+const { samplesRunning } = submissionStore.trial
+const { problem } = storeToRefs(useProblemStore())
+
+// 和工具栏同一个条件：Judge0 跑不了 SQL，流程图没得跑，没有例子的题不给
+const canRunSamples = computed(
+  () =>
+    !!problem.value?.samples.length &&
+    codeStore.code.language !== "Flowchart" &&
+    codeStore.code.language !== "SQL",
+)
+</script>
+
+<template>
+  <div class="mobile-actions">
+    <n-button
+      v-if="canRunSamples"
+      size="large"
+      class="action"
+      :loading="samplesRunning"
+      :disabled="!codeStore.code.value.trim()"
+      @click="submissionStore.runSamples()"
+    >
+      运行例子
+    </n-button>
+    <div class="action">
+      <SubmitCode size="large" block />
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.mobile-actions {
+  position: fixed;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  z-index: 10;
+  display: flex;
+  gap: 10px;
+  padding: 8px 16px calc(8px + env(safe-area-inset-bottom));
+  background-color: v-bind("theme.bodyColor");
+  border-top: 1px solid rgba(128, 128, 128, 0.2);
+  backdrop-filter: blur(8px);
+}
+
+.action {
+  flex: 1;
+  min-width: 0;
+}
+
+.mobile-actions :deep(.n-button) {
+  height: 44px;
+}
+</style>