fix(stats): address Claude review follow-ups

backlog/tasks/task-205 - Address-PR-19-Claude-frontend-review-follow-ups.md

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+---
+id: TASK-205
+title: 'Address PR #19 Claude frontend review follow-ups'
+status: Done
+assignee:
+  - codex
+created_date: '2026-03-20 02:41'
+updated_date: '2026-03-20 02:46'
+labels: []
+milestone: m-1
+dependencies: []
+references:
+  - stats/src/components/vocabulary/VocabularyTab.tsx
+  - stats/src/hooks/useSessions.ts
+  - stats/src/hooks/useTrends.ts
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Assess Claude's latest PR #19 review, apply any valid frontend fixes from that review batch, and verify the stats dashboard behavior stays unchanged aside from the targeted performance and error-handling improvements.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 VocabularyTab avoids recomputing expensive known-word and summary aggregates on unrelated rerenders while preserving current displayed values.
+- [x] #2 useSessions and useSessionDetail normalize rejected values into stable string errors without throwing from the catch handler.
+- [x] #3 Targeted tests cover the addressed review items and pass locally.
+- [x] #4 Any user-facing docs remain accurate after the changes.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add focused tests that fail on the current branch for the two valid Claude findings: render-time aggregate recomputation in VocabularyTab and unsafe non-Error rejection handling in useSessions/useSessionDetail.
+2. Update VocabularyTab to memoize the expensive summary and known-word aggregate calculations off the existing filteredWords/kanji/knownWords inputs without changing rendered values.
+3. Normalize hook error handling to convert unknown rejection values into stable strings, matching the existing useTrends pattern.
+4. Run the targeted stats/frontend test lane, verify no docs changes are needed, and record results in task notes.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Validated Claude's latest PR #19 review comment from 2026-03-20 and narrowed it to two valid frontend follow-ups: memoized VocabularyTab aggregates and non-Error-safe session hook error handling.
+
+Added focused regression tests in stats/src/lib/vocabulary-tab.test.ts and stats/src/hooks/useSessions.test.ts before patching the implementation.
+
+Verification: `cd stats && bun test src/lib/vocabulary-tab.test.ts src/hooks/useSessions.test.ts` passed; `bun run format:check:stats` passed.
+
+Project-native verifier (`.agents/skills/subminer-change-verification/scripts/verify_subminer_change.sh --lane core ...`) passed root `bun run typecheck` and failed at `bun run test:fast` due an unrelated existing failure in `scripts/update-aur-package.test.ts` (`mapfile: command not found`). Artifact: `.tmp/skill-verification/subminer-verify-20260319-194525-vxVD9V`.
+
+No user-facing docs changes were needed because the fixes only affect render-time memoization and error normalization.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Assessed Claude's latest PR #19 review and applied the two valid follow-ups. `stats/src/components/vocabulary/VocabularyTab.tsx` now memoizes `buildVocabularySummary(filteredWords, kanji)` and the known-word count so unrelated rerenders do not rescan the filtered vocabulary list. `stats/src/hooks/useSessions.ts` now exports a small `toErrorMessage` helper and uses it in both `useSessions` and `useSessionDetail`, preventing `.catch()` handlers from throwing when a promise rejects with a non-`Error` value.
+
+Added targeted regressions in `stats/src/lib/vocabulary-tab.test.ts` and `stats/src/hooks/useSessions.test.ts` to lock in the memoization shape and error normalization behavior. Verification passed for `cd stats && bun test src/lib/vocabulary-tab.test.ts src/hooks/useSessions.test.ts` and `bun run format:check:stats`. The repo-native verification wrapper for the classified `core` lane also passed root `bun run typecheck`, but `bun run test:fast` is currently blocked by an unrelated existing failure in `scripts/update-aur-package.test.ts` (`mapfile: command not found`); artifacts are recorded under `.tmp/skill-verification/subminer-verify-20260319-194525-vxVD9V`.
+<!-- SECTION:FINAL_SUMMARY:END -->