feat: UI/UX improvements pass + fix photo 404 in Docker

Photos:
- Serve uploads via /api/photos/[filename] from UPLOAD_DIR env var
- Default UPLOAD_DIR=/data/uploads (mount as Docker volume for persistence)
- Upload route creates dir on demand, no root-permission issue
- Dockerfile creates /data/uploads owned by nextjs user

Growth:
- Show chart with single measurement (was hidden until 2+ points)
- PC label expanded to "PC — Périmètre crânien (cm)"
- WHO percentile ⓘ tooltip explaining P50/P3-P97
- Date field defaults to today in add-measurement form

Stats:
- Heatmap legend shows actual counts (0 1 2 3 4+) not just Moins/Plus
- Sleep timeline labeled "Fenêtre 19h – 11h"

Timeline:
- Filter type persisted in URL param (?type=...) — survives navigation
- Inline note save shows ✓ flash after blur
- Photo thumbnails open in new tab

Photos:
- Lightbox wraps: next on last → first, prev on first → last

Search:
- Result count header "X événements · Y notes"
- Event results link to /timeline?date=...&type=...
- Note results link to /notes?date=...

Notes:
- Unsaved indicator (amber dot) while textarea dirty, green ✓ after save

Milestones:
- Sort by date ascending within each month group

Settings:
- Pushover key field has eye toggle (show/hide)
- API key delete shows confirmation with key name
- All sections wrapped in collapsible SectionCard accordion

Dashboard:
- Empty state callout for new users with no events today

Event modal:
- Milk lot selection shows "Sélectionnez un lot — il sera marqué comme utilisé"
- Photo upload validates 5MB client-side before sending
- Timer resume shows "Reprise du chronomètre" when loaded from localStorage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -43,6 +43,10 @@ COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
 COPY --from=builder --chown=nextjs:nodejs /app/src/generated ./src/generated
 COPY --from=builder /app/prisma ./prisma
 
+# Upload directory — mount a volume here for persistence: -v /host/uploads:/data/uploads
+RUN mkdir -p /data/uploads && chown nextjs:nodejs /data/uploads
+ENV UPLOAD_DIR=/data/uploads
+
 USER nextjs
 
 EXPOSE 3000