[签字门] #1140 fix(desktop): 修复会话内 Ctrl/Cmd+F 搜索无关或隐藏内容

PR #1140 触发产品签字门。

**触发原因**：命中 UI 路径（MessageStream.tsx / FindInPageBar.tsx / SessionSearchBar.tsx / globals.css / colors.ts 等），涉及聊天区搜索交互与样式改动。

**需要**：放行名单成员确认产品行为符合预期后在 PR 上 Approve 即放行。如需修改请 Request Changes，作者改完后重新 Approve。

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关联 PR:#1140(作者 @xushixd);本 issue 由 review-pr 流程自动创建,用于先讨论该 PR 涉及的产品 / UI 变更,维护者确认后 PR 会恢复推进。

Start by reviewing PR #1140 and the referenced UI files: MessageStream.tsx, FindInPageBar.tsx, SessionSearchBar.tsx, globals.css, and colors.ts. Verify the Ctrl/Cmd+F behavior for irrelevant or hidden chat content against the product expectations; done means the review is approved or requested changes are recorded on the PR.

Written by the indexing model from the issue text.