Add page-grouped search results with time decay

[twaugh]

Summary

• Page grouping: when multiple blocks from the same page match a query, they collapse into an expandable page group showing the best score and block count. Journal pages are never grouped.
• Time decay: journal results get bounded exponential decay (floor 0.85, scale 180 days) so recent entries rank above older ones.
• Adaptive thresholds: short queries (1-3 words) require 4 blocks to group; longer queries (8+) require 6, via linear interpolation.
• Density bonus: harmonic decay (1/i weighting) with W_MAX=0.15 prevents pages with a few mediocre matches from outranking better individual results.
• Overfetch: 2x topK candidates fetched for reranking, balancing grouping accuracy with avoiding excessive grouping.
• Page group interactions: click to expand/collapse, Shift+click/Enter opens page in sidebar, Ctrl+C copies [[page reference]], copy button on hover.

Test plan

• npx vitest run — 74 tests pass (18 new ranking tests)
• npm run build succeeds
• Manual test: short query shows page groups, long query shows individual blocks
• Manual test: page groups expand/collapse, keyboard nav works
• Manual test: old journal entries rank slightly lower than recent ones
• Manual test: journal filter hides/shows grouped results correctly
• Recreate demo gif

🤖 Generated with Claude Code