Add Pretext for reflow-free text measurement

[hudsonhrh]

Summary

• Adds @chenglou/pretext and a new usePretext.js hooks module with cached useTextHeight, useTextTruncation, useAutoTextHeight, and useBatchTextHeights utilities
• Replaces arbitrary character-count truncation with width-aware Pretext measurement in TaskCard, HistoryCard, VoteCard, and OrgStructureCard
• Pre-sizes typing animation containers in SpeechBubble to eliminate layout thrashing during character-by-character rendering
• Uses Pretext-estimated scroll height in ConversationLog for instant initial scroll without forced DOM reflow

Test plan

• Verify TaskCard descriptions truncate correctly at different viewport widths
• Verify HistoryCard and VoteCard proposal descriptions show proper ellipsis
• Verify OrgStructureCard role badges truncate long names
• Verify SpeechBubble typing animation doesn't cause layout jumping
• Verify ConversationLog auto-scrolls to bottom on load and new messages

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