Spark

feat(app): add two-step confirmation for archive session

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### What does this PR do? Closes #7886 ### How did you verify your code works? Add two-step confirmation when archiving sessions in WebUI sidebar to prevent accidental data loss....

feat(app): add two-step confirmation for archive session

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Single-click archive in sidebar can cause accidental data loss. TUI already has confirmation for delete: Request: Add similar two-step confirmation to WebUI.

bug(webui): archived session "flashes back" due to SSE race condition

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## Problem When archiving a session in WebUI sidebar, the session briefly disappears then reappears (~100ms later). ## Possible Cause The `session.archive` API may trigger a `session.updated` SSE event before...

feat(app): add /thinking command to toggle reasoning visibility

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### What does this PR do? Adds `/thinking` slash command to WebUI, matching TUI behavior for toggling thinking/reasoning block visibility. - Default: visible (same as TUI) - Command toggles show/hide...

WebUI: thinking/reasoning blocks hidden after response completion, no way to show them

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## Problem In the TUI, users can toggle thinking/reasoning blocks visibility with `/thinking` command (default: visible). In the WebUI, thinking blocks are **always hidden** after response completion with no way...