Add direct bar quote reaction via keyword matching

[shimpeiws]

Summary

Implements issue #212 by adding a keyword-matching path that returns bar text as reactions without going through the LLM. Fires before the LLM call and is gated behind the existing barQuote feature flag (default: OFF).

• Build inverted index from bars for O(k) keyword lookups
• Add probability gate (40% skip) and dedup checking
• Integrate into reaction service before LLM path

Changes

• src/services/llm/bar-index.ts: New inverted index mapping non-stopword tokens to bars (>100 chars). Threshold: ja=1 match, en=2 matches.
• src/services/llm/bar-quote.ts: Orchestrates bar quote attempts with probability gate and dedup via isDuplicate().
• src/services/reaction-service.ts: Added setFeatures() method, bar index build in setVocabulary(), bar quote check before LLM call in generateReaction().
• src/index.tsx: Pass initial features to reaction service at startup.
• src/ui/context/ConfigContext.tsx: Propagate feature toggle to reaction service on change.
• src/services/llm/bar-index.test.ts: 10 tests covering index construction, thresholds, stopword filtering, edge cases.
• src/services/llm/bar-quote.test.ts: 8 tests covering probability gate, dedup, no-match, boundary cases.

Test plan

• Feature flag OFF (default): bar quote never fires, LLM path only
• Feature flag ON via config UI (c -> f -> toggle): bar quotes appear for keyword matches
• Bar quotes are deduplicated against recent reactions
• Probability gate prevents bar quotes from being too frequent
• All existing tests continue to pass

Resolves #212