chore: remove the dashboard's pre-snapshot feed

NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false built a dashboard that read its
tiles from two analytics slices, the widget layout, the mood analytics
and the thresholds instead of the one snapshot cell. It took a rebuilt
image to reach, and the snapshot has been the only path the published
image runs since it became the default. The flag, its readers, the
legacy reads in the dashboard hook and tile context, the slim and thick
merge helper, the build-time toggle notes in .env.production.example and
the self-hosting guide, and the tests that pinned the legacy wiring go.

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 336,
-  "total": 109385,
+  "total": 109383,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 275,
@@ -38,7 +38,7 @@
     "app/api/record-settings/[family]/route.ts :: apiHandler(arg) #2": 262,
     "app/api/user/ai-provider/route.ts :: apiHandler(arg) #2": 250,
     "app/api/withings/callback/route.ts :: apiHandler(arg)": 247,
-    "app/auth/login/page.tsx :: LoginPage": 569,
+    "app/auth/login/page.tsx :: LoginPage": 567,
     "app/auth/register/page.tsx :: RegisterPage": 285,
     "app/coach/conversations/page.tsx :: CoachConversationsBody": 249,
     "app/coach/plans/page.tsx :: CoachPlansBody": 216,