feat(chat): add copy button for AI messages

[sparkleMing]

Summary

在 AgentChatDialog 的每条 AI 回复气泡下方新增一键复制按钮，点击即可将原始 Markdown 文本复制到剪贴板。

Changes

• AI 消息气泡外部下方左侧新增 copy icon（18px outlined 风格）
• 点击后图标动画切换为 ✓，2 秒后恢复
• Toast 提示「已复制到剪贴板」
• 流式输出中不显示按钮，避免干扰

Testing

• 手动测试：发送消息后 AI 回复完成，点击复制按钮，粘贴验证内容为原始 Markdown 源文本
• 流式输出期间按钮不可见

Closes #156

[github-actions]

PR Preflight Summary / PR 预检汇总

中文

• 统一结论：低风险：两个预检均已完成，质量预检通过，可走普通手动合并流程。
• Policy preflight：低风险。未命中打回或高风险规则，仅有 1 条警告供 review 参考。
• Flutter quality：通过。Analyzer 和 test baseline 均未发现新增问题。
• PR head：76c7cd455e2a860816ed7e8ab64f1d35ee9ac973
• Policy run：26076216050
• Flutter run：26076216034

English

• Combined result: Low risk: both preflights completed and quality passed; use the normal manual merge flow.
• Policy preflight: LOW RISK. No blocking or high-risk policy signal; 1 warning(s) remain for review context.
• Flutter quality: PASS. Analyzer and test baselines found no newly introduced issue.
• PR head: 76c7cd455e2a860816ed7e8ab64f1d35ee9ac973
• Policy run: 26076216050
• Flutter run: 26076216034

PR Policy Preflight / PR 规则预检

PR Policy Preflight / PR 规则预检

中文

• 判定：低风险
• 变更文件数：1
• 变更行数：161
• Diff 是否截断：false

规则命中

• 警告 sensitive-keyword lib/ui/chat/widgets/agent_chat_dialog.dart: UserStorage 引用发生变化。

English

• Decision: LOW RISK
• Changed files: 1
• Changed lines: 161
• Diff truncated: false

Findings

• warn sensitive-keyword lib/ui/chat/widgets/agent_chat_dialog.dart: UserStorage reference changed.

PR Flutter Quality / Flutter 质量预检

PR Flutter Quality / Flutter 质量预检

中文

• 总体：通过
• Analyzer baseline：通过
• Test baseline：通过

English

• Overall: PASS
• Analyzer baseline: PASS
• Test baseline: PASS

Flutter Analyzer Baseline

• Base issues: 324
• PR issues: 324
• New issues: 0

No new analyzer issues introduced by this PR.

Flutter Test Baseline

• Base failures: 3
• PR failures: 3
• New failures: 0

No new Flutter test failures introduced by this PR.