Add PDF export option for next-gen thread pages

[ekassos]

Summary

• add a next-gen-only print/PDF action that loads all thread messages and prepares the UI for export
• expand reasoning accordions on demand and keep code/output accordions open during print preparation
• add print-friendly layout rules to keep thread content together on exported pages

Testing

• pnpm test index.test.ts

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Codex Task

[sentry]

Bug: expandContentForPrint is not reset after print dialog closes, leaving UI elements permanently expanded.
_{Severity: HIGH | Confidence: High}

🔍 Detailed Analysis

The expandContentForPrint variable is set to true when the print handler is invoked but lacks a mechanism to reset it to false. This oversight causes AccordionItem components (lines 1027, 1067, 1087, 1105) and ReasoningCallItem components (line 1064) to remain permanently expanded after the print dialog is closed, resulting in a broken UI state that persists until the page is manually refreshed.

💡 Suggested Fix

Add an onafterprint event listener to reset expandContentForPrint to false after window.print() is called, ensuring the UI state is properly restored.

🤖 Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: web/pingpong/src/lib/components/ThreadDetailPage.svelte#L230-L236

Potential issue: The `expandContentForPrint` variable is set to `true` when the print
handler is invoked but lacks a mechanism to reset it to `false`. This oversight causes
`AccordionItem` components (lines 1027, 1067, 1087, 1105) and `ReasoningCallItem`
components (line 1064) to remain permanently expanded after the print dialog is closed,
resulting in a broken UI state that persists until the page is manually refreshed.

_{Did we get this right? 👍 / 👎 to inform future reviews.
Reference ID: 4357959}