Add body weight logging with a trend on the dashboard

No way existed to log body weight over time - only the single
self-reported value from the signup profile - and the dashboard showed
session activity but nothing about physical progress, which is usually
the main reason people keep using a fitness app.

Adds a WeightLog model (one entry per user per day, re-logging the same
day updates it rather than duplicating), a quick-add form and trend
summary on the dashboard, and delete support.

Found and fixed a real bug along the way: the first implementation used
WeightLog::updateOrCreate() with a raw ['logged_at' => $date] match
array, which silently failed to find the same-day entry because the
`date` cast stores full datetime strings ('2026-01-01 00:00:00') while
the match array holds a plain 'Y-m-d' string - so re-logging the same
day hit a unique-constraint violation instead of updating. Fixed by
using a whereDate() lookup instead, which is comparison-format-safe.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Http/Controllers/DashboardController.php

@@ -49,13 +49,21 @@ class DashboardController extends Controller
             ->groupBy(function ($s) { return $s->created_at->format('Y-m-d'); })
             ->map->count();
 
+        // Body weight trend
+        $weightLogs = $user->weightLogs()->orderBy('logged_at')->get(['id', 'weight_kg', 'logged_at']);
+        $weightChange = $weightLogs->count() >= 2
+            ? round($weightLogs->last()->weight_kg - $weightLogs->first()->weight_kg, 2)
+            : null;
+
         return view('dashboard', compact(
             'workoutsCount',
             'sessionsCount',
             'completedSessions',
             'completionRate',
             'recentSessions',
-            'dailyActivity'
+            'dailyActivity',
+            'weightLogs',
+            'weightChange'
         ));
     }
 }