UX cleanup + consensus engine hardening

[msitarzewski]

Summary

• Thread detail collapse defaults — all sections collapsed on load except decision (with dissent collapsed inside)
• max_tokens 4096 -> 16384 for propose/challenge/revise phases, with token budget communicated to LLMs in system prompts
• Truncation detection — finish_reason checked after each phase, truncated flag sent via WebSocket, amber warning in PhaseCard UI
• Cross-provider challenger selection — prefers one model per different provider for genuine intellectual diversity
• Glass-styled export dropdowns — both ConsensusComplete and ExportMenu menus match design system
• PDF export crash fix — missing bold-italic (BI) TTF font variant caused crash when dissent markdown had bold text

Test plan

• Load a completed thread — all rounds collapsed, decision open, dissent inside decision collapsed
• Run a consensus question — verify challengers are from different providers when available
• Check export dropdown styling matches glass design
• Export PDF with dissent containing bold markdown — no crash
• Verify long responses don't truncate (16384 token budget)

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