Track who paid each depense, with a "Commun" (shared) fallback

Adds a "payé par" field to transactions and recurring transactions:
either a household member or null (shared/common expense). No per-
member account balances — this is purely a "who paid" label, the
household's overall balance is unchanged and unaffected by any
filtering.

- paid_by_user_id nullable FK on transactions and recurring_transactions
- Suivi: "payé par" selector on both entry forms (desktop dropdown,
  mobile buttons), a filter above the transaction table, a "payé par"
  column, and a summary tile showing this month's depenses grouped
  by payer (including Commun)
- Recurring transactions carry paid_by_user_id onto the transaction
  they generate when confirmed
- Running balance is computed from all transactions regardless of
  the payer filter, so it never gets skewed by the active filter

11 new Pest tests (assignment, common fallback, non-member rejection,
filter isolation from the balance calc, per-payer summary, recurring
propagation). 69/69 total pass, Pint clean, verified end-to-end on
production data (desktop + mobile, test row created and removed).

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

app/Models/RecurringTransaction.php

@@ -11,7 +11,7 @@ use Illuminate\Database\Eloquent\Relations\HasMany;
 class RecurringTransaction extends Model
 {
     protected $fillable = [
-        'household_id', 'category_id', 'name', 'amount', 'day_of_month',
+        'household_id', 'category_id', 'paid_by_user_id', 'name', 'amount', 'day_of_month',
         'active', 'last_generated_year', 'last_generated_month',
     ];
 
@@ -33,6 +33,11 @@ class RecurringTransaction extends Model
         return $this->belongsTo(Category::class);
     }
 
+    public function paidBy(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'paid_by_user_id');
+    }
+
     public function transactions(): HasMany
     {
         return $this->hasMany(Transaction::class);