Add a community workout library

Workouts were private to their owner with no way to browse or copy
another user's - FavoriteController::toggleWorkout even had a comment
anticipating sharing ("users can still favorite others' workouts if
shared later") that was never followed up on.

Adds an is_public flag workouts can be toggled to from their own page,
a /workouts/community browse+search page, and a copy action that
duplicates a public workout (including all its exercises/sets/reps)
into the copier's own library.

Making this work required splitting workout authorization into two
tiers: authorizeView() (owner, the assigning trainer, or anyone if the
workout is public - read-only) and the existing authorizeWorkout()
(owner or assigning trainer - can add/edit/remove exercises). Deletion
stays strictly owner-only as before. The show page now hides the
"Add Exercise" panel and per-exercise edit/remove controls for viewers
who can see but not manage a workout.

Found and fixed a real Blade compiler quirk via live testing: `@if`
directly after a word character with no space (`exercises@if(...)`)
doesn't compile - Blade's directive regex treats it as email-like text
and leaves it literal - while the matching `@endif` still compiles,
producing invalid mismatched PHP. Fixed by adding a space.

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

app/Models/Workout.php

@@ -21,12 +21,14 @@ class Workout extends Model
         'week_number',
         'day_number',
         'assigned_by_trainer_id',
+        'is_public',
     ];
 
     protected $casts = [
         'estimated_minutes' => 'integer',
         'week_number' => 'integer',
         'day_number' => 'integer',
+        'is_public' => 'boolean',
     ];
 
     public function user(): BelongsTo