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>

database/migrations/2026_08_01_061025_add_paid_by_user_id_to_transactions_table.php

@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            // Null = dépense commune, non attribuée à un membre en particulier.
+            $table->foreignId('paid_by_user_id')->nullable()->after('user_id')->constrained('users')->nullOnDelete();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->dropConstrainedForeignId('paid_by_user_id');
+        });
+    }
+};

database/migrations/2026_08_01_061052_add_paid_by_user_id_to_recurring_transactions_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('recurring_transactions', function (Blueprint $table) {
+            $table->foreignId('paid_by_user_id')->nullable()->after('category_id')->constrained('users')->nullOnDelete();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('recurring_transactions', function (Blueprint $table) {
+            $table->dropConstrainedForeignId('paid_by_user_id');
+        });
+    }
+};