Replace capped exercise picker with a real search combobox

The workout builder's "add exercise" dropdown was Exercise::orderBy
('name')->limit(50) with no search. Harmless with a handful of seeded
exercises, but once the library is populated with real data (1000+),
users could only ever add exercises starting around A-D - the core
"build a workout" flow would be effectively broken.

Replaces it with a debounced Alpine.js combobox backed by a new
GET /workouts/{workout}/exercises/search endpoint (owner-only, JSON,
capped at 20 results, respects each user's custom exercise aliases).

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

routes/web.php

@@ -46,6 +46,7 @@ Route::middleware('auth')->prefix('/workouts')->group(function () {
     Route::delete('/{workout}', [\App\Http\Controllers\WorkoutController::class, 'destroy'])->name('workouts.destroy');
 
     // Manage exercises within a workout
+    Route::get('/{workout}/exercises/search', [\App\Http\Controllers\WorkoutController::class, 'searchExercises'])->name('workouts.exercises.search');
     Route::post('/{workout}/exercises', [\App\Http\Controllers\WorkoutController::class, 'addExercise'])->name('workouts.exercises.add');
     Route::patch('/{workout}/exercises/{item}', [\App\Http\Controllers\WorkoutController::class, 'updateExercise'])->name('workouts.exercises.update');
     Route::delete('/{workout}/exercises/{item}', [\App\Http\Controllers\WorkoutController::class, 'removeExercise'])->name('workouts.exercises.remove');