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>

routes/web.php

@@ -30,6 +30,10 @@ Route::middleware('auth')->group(function () {
 
     // Exercise alias (per-user rename)
     Route::post('/exercises/{exercise}/alias', [\App\Http\Controllers\ExerciseAliasController::class, 'store'])->name('exercises.alias');
+
+    // Body weight log
+    Route::post('/weight-logs', [\App\Http\Controllers\WeightLogController::class, 'store'])->name('weight-logs.store');
+    Route::delete('/weight-logs/{weightLog}', [\App\Http\Controllers\WeightLogController::class, 'destroy'])->name('weight-logs.destroy');
 });
 
 Route::prefix('/exercises')->group(function () {