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/Models/WeightLog.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class WeightLog extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'user_id',
+        'weight_kg',
+        'logged_at',
+    ];
+
+    protected $casts = [
+        'weight_kg' => 'decimal:2',
+        'logged_at' => 'date',
+    ];
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
+}