实现pdf中划句子ai伴读的功能，修复pdf高亮显示的问题

addon/locale/en-US/addon.ftl

@@ -101,6 +101,16 @@ lecture-sentence-list-title = Sentences (original, then explanation)
 lecture-sentence-label = P{ $p }·S{ $s }
 lecture-sentence-list-hint = Sentences in this paragraph
 lecture-pdf-sync-hint = The matching passage is highlighted in the PDF reader while playing.
+lecture-explain-selection = Explain & read selection
+lecture-explain-selection-short = Explain
+lecture-selection-empty = Select a sentence in the PDF first
+lecture-selection-explaining = Generating explanation for selection…
+lecture-selection-done = Selection explanation ready
+lecture-selection-no-item = Cannot resolve the current PDF item
+lecture-selection-title = Selection explain
+lecture-selection-syntax = Syntax / terms
+lecture-selection-context = Context role
+lecture-selection-replay = Replay explanation
 lecture-playing-en = Playing: original
 lecture-playing-zh = Playing: translation
 lecture-status-ready-step2 = Ready: { $count } paragraphs + note synced

addon/locale/zh-CN/addon.ftl

@@ -101,6 +101,16 @@ lecture-sentence-list-title = 逐句精读（先读原文，再听讲解）
 lecture-sentence-label = 段{ $p }·句{ $s }
 lecture-sentence-list-hint = 本段各句
 lecture-pdf-sync-hint = 播放时会在 PDF 阅读器中高亮对应原文。
+lecture-explain-selection = 解释朗读选区
+lecture-explain-selection-short = 解释朗读
+lecture-selection-empty = 请先在 PDF 中选中要解释的句子
+lecture-selection-explaining = 正在生成选区讲解…
+lecture-selection-done = 选区讲解完成
+lecture-selection-no-item = 无法识别当前 PDF 条目
+lecture-selection-title = 选区讲解
+lecture-selection-syntax = 句法/术语
+lecture-selection-context = 上下文作用
+lecture-selection-replay = 重播讲解
 lecture-playing-en = 正在播放：原文朗读
 lecture-playing-zh = 正在播放：中文讲解
 lecture-status-ready-step2 = 备课完成：{ $count } 段逐段精读可播放，笔记已同步