perf(reactionpicker): fold candidate names once - #177

Summary

• fold each reaction-picker candidate name once and reuse it for prefix and substring checks
• cover substring selection through the exported SetCustomEmoji, Open, and HandleKey seam, observing the selected ReactionResult
• benchmark a full-scan query over 10,000 custom emoji through the same input seam

This is the first of two independent performance follow-ups requested after PR #174 / issue #165.

Benchmark

Apple M3 Max, darwin/arm64:

go test ./internal/ui/reactionpicker -run '^$' -bench '^BenchmarkFilterByQuery$' -benchmem -count=6 -benchtime=500ms

Observed median: 604,271 -> 341,907 ns/op (43.4% lower). Memory remained 0 B/op, 0 allocs/op before and after.

Testing

• go test ./internal/ui/reactionpicker -run 'TestSubstringSearchSelectsCustomEmoji|TestFilterByQuery'
• go test -race ./internal/ui/reactionpicker
• benchmark command above

Refs #165