feat(typesetting): Wave 1 - 对齐修复+底部对齐、封面页字号豁免、字号UI改造

- 修复对齐按钮缺少click handler的缺陷，新增事件委托
- 三态对齐切换: 居顶(↑) -> 居中(↕) -> 居底(↓)
- 封面页和尾页通过CSS变量重置不受全局fontScale影响
- 字号控制从slider改为dropdown+自定义输入，预设0.5/0.8/1.0/1.2/1.5/2.0

public/static/css/phone-image-templates.css

@@ -112,6 +112,7 @@
 
 /* --- 封面页/首页 --- */
 .page-cover {
+    --pi-font-scale: 1 !important;
     display: flex;
     flex-direction: column;
     justify-content: center;
@@ -335,6 +336,7 @@
 
 /* --- 总结页/尾页 --- */
 .page-summary {
+    --pi-font-scale: 1 !important;
     display: flex;
     flex-direction: column;
     justify-content: center;
@@ -417,6 +419,12 @@
     justify-content: center;
 }
 
+.page-body.valign-bottom .page-content {
+    display: flex;
+    flex-direction: column;
+    justify-content: flex-end;
+}
+
 /* ============================================
    Content Flow (中间栏内容流)
    所有元素按实际尺寸纵向排列