feat(stats): add v1 immersion stats dashboard (#19)

docs/architecture/stats-trends-data-flow.md

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+# Stats Trends Data Flow
+
+read_when: touching stats trend charts, changing stats API payloads, or debugging dashboard performance
+
+## Summary
+
+Trend charts now consume one chart-oriented backend payload from `/api/stats/trends/dashboard`.
+
+## Why
+
+- remove repeated client-side dataset rebuilding in `TrendsTab`
+- collapse multiple network round-trips into one request
+- keep heavy chart shaping close to tracker/query logic
+
+## Data Sources
+
+- rollup-backed:
+  - activity charts
+  - cumulative watch/cards/tokens/sessions trends
+  - per-anime watch/cards/tokens/episodes series
+- session-metric-backed:
+  - lookup trends
+  - lookup rate trends
+  - watch-time by day-of-week/hour
+- vocabulary-backed:
+  - new-words trend
+
+## Metric Semantics
+
+- subtitle-count stats now use Yomitan merged-token counts as the source of truth
+- `tokensSeen` is the only active subtitle-count metric in tracker/session/rollup/query paths
+- no whitespace/CJK-character fallback remains in the live stats path
+
+## Contract
+
+The stats UI should treat the trends payload as chart-ready data. Presentation-only work in the client is fine, but rebuilding the main trend datasets from raw sessions should stay out of the render path.
+
+For session detail timelines, omitting `limit` now means "return the full retained session telemetry/history". Explicit `limit` remains available for bounded callers, but the default stats UI path should not trim long sessions to the newest 200 samples.