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>

src/app/(app)/dashboard/page.tsx

@@ -600,6 +600,17 @@ export default function DashboardPage() {
         })}
       </div>
 
+      {/* Empty state for new users */}
+      {todayEvents.length === 0 && events.filter((e) => !e.endedAt && EVENT_CONFIG[e.type]?.hasTimer).length === 0 && (
+        <div className="bg-indigo-50 dark:bg-indigo-950 border border-indigo-100 dark:border-indigo-900 rounded-xl p-5 mb-4 flex items-start gap-3">
+          <span className="text-2xl">👶</span>
+          <div>
+            <p className="text-sm font-semibold text-indigo-800 dark:text-indigo-200">Bienvenue sur Grow !</p>
+            <p className="text-xs text-indigo-600 dark:text-indigo-400 mt-0.5">Commencez par enregistrer un repas, une couche ou un sommeil via les boutons ci-dessous.</p>
+          </div>
+        </div>
+      )}
+
       {/* Quick log */}
       <div className="space-y-4">
         {QUICK_LOG_GROUPS.map((group) => (