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

backlog/tasks/task-193 - Fix-session-chart-event-popup-position-drift.md

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+---
+id: TASK-193
+title: Fix session chart event popup position drift
+status: Done
+assignee:
+  - Codex
+created_date: '2026-03-17 23:55'
+updated_date: '2026-03-17 23:59'
+labels:
+  - stats
+  - ui
+  - bug
+milestone: m-1
+dependencies: []
+references:
+  - stats/src/components/sessions/SessionDetail.tsx
+  - stats/src/components/sessions/SessionEventOverlay.tsx
+  - stats/src/lib/session-events.ts
+priority: medium
+ordinal: 105600
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+
+Fix the session timeline event popup trigger positions so hover markers stay aligned with the underlying chart event lines across the full visible time range.
+
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+
+- [x] #1 Event popup triggers stay horizontally aligned with chart event lines from session start through session end.
+- [x] #2 Alignment logic uses the rendered chart plot area rather than guessed container percentages.
+- [x] #3 Regression coverage locks the marker-position projection math.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+
+1. Add a failing regression test for marker-position projection with chart offsets.
+2. Capture the rendered plot box from Recharts and pass it into the overlay.
+3. Position overlay markers in plot-area pixels, rerun targeted stats verification, then record the result.
+<!-- SECTION:PLAN:END -->
+
+## Outcome
+
+<!-- SECTION:OUTCOME:BEGIN -->
+
+Completed. Session event hover markers now read the actual Recharts plot-area offset and width, then project marker X positions into plot-area pixels instead of full-container percentages. That keeps popup triggers aligned with the underlying reference lines across long session timelines.
+
+Verification:
+
+- `bun test stats/src/lib/session-events.test.ts stats/src/lib/session-detail.test.tsx stats/src/components/sessions/SessionEventPopover.test.tsx`
+- `cd stats && bun run build`
+- `bun x prettier --check 'stats/src/components/sessions/SessionDetail.tsx' 'stats/src/components/sessions/SessionEventOverlay.tsx' 'stats/src/lib/session-events.ts' 'stats/src/lib/session-events.test.ts' 'backlog/tasks/task-193 - Fix-session-chart-event-popup-position-drift.md'`
+- `bun run typecheck:stats` still fails on pre-existing unrelated errors in `src/components/anime/AnilistSelector.tsx`, `src/components/library/LibraryTab.tsx`, `src/lib/reading-utils.test.ts`, `src/lib/reading-utils.ts`, `src/lib/vocabulary-tab.test.ts`, and `src/lib/yomitan-lookup.test.tsx`
+
+<!-- SECTION:OUTCOME:END -->