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>

lang/en/app.php

@@ -231,6 +231,19 @@ return [
         'activity_30d_help' => 'Number of sessions per day (higher is better).',
     ],
 
+    'weight_log' => [
+        'title' => 'Body Weight',
+        'change_since_first' => 'Change since your first entry:',
+        'weight_kg' => 'Weight (kg)',
+        'date' => 'Date',
+        'log' => 'Log',
+        'none' => 'No weight entries yet. Log your first one to start tracking your trend.',
+        'saved' => 'Weight logged',
+        'deleted' => 'Entry deleted',
+        'delete' => 'Delete',
+        'confirm_delete' => 'Delete this entry?',
+    ],
+
     'admin' => [
         'title' => 'Admin Dashboard',
         'subtitle' => 'Manage content and see overall site statistics.',