fix(messages): dynamic addresses, branding accent, compose/sync, per-identity SMTP

Addresses dev-test feedback:
- Sidebar + reading-pane addresses (rechnungen@ / hello@ / no-reply@) are now
  read from the mail config via GET /admin/email/identities, not hardcoded.
- Highlight/selection now uses the branding accent (bg-accent-soft /
  text-on-accent-soft / accent-dark) instead of hardcoded blue, so it follows
  the admin's CI colour like the sidebar.
- Header gains "New message" (compose) and "Sync" (poll mailboxes now) buttons.
- Composer modal enlarged (920px, taller editable body).
- Customer mailbox (hello@) now has BOTH incoming (IMAP) and outgoing (SMTP)
  settings — migration 156 adds smtp_* + from_* to mail_accounts;
  emailProcessor.sendRawEmail takes an accountKey and sends via that mailbox's
  SMTP identity (falls back to the global from). Manual/reply sends from the
  Messages UI use the 'customers' identity, so replies come from hello@.

Frontend build + migration boot (156) verified.

backend/migrations/core/156_mail_accounts_smtp.js

@@ -0,0 +1,34 @@
+/**
+ * Messages Phase 3 follow-up — outgoing (SMTP) settings per mail account.
+ *
+ * The customer mailbox (hello@) needs BOTH incoming (IMAP, migration 154) and
+ * outgoing (SMTP) config, so replies to customers send from hello@ instead of
+ * the global no-reply@ identity. All additive/guarded.
+ */
+exports.up = async function up(knex) {
+  const cols = [
+    ['smtp_host', (t) => t.string('smtp_host', 255)],
+    ['smtp_port', (t) => t.integer('smtp_port')],
+    ['smtp_secure', (t) => t.boolean('smtp_secure').defaultTo(false)],
+    ['smtp_user', (t) => t.string('smtp_user', 255)],
+    ['smtp_pass', (t) => t.string('smtp_pass', 512)],
+    ['from_email', (t) => t.string('from_email', 255)],
+    ['from_name', (t) => t.string('from_name', 120)],
+  ];
+  for (const [name, add] of cols) {
+    // eslint-disable-next-line no-await-in-loop
+    const has = await knex.schema.hasColumn('mail_accounts', name);
+    // eslint-disable-next-line no-await-in-loop
+    if (!has) await knex.schema.alterTable('mail_accounts', add);
+  }
+};
+
+exports.down = async function down(knex) {
+  const cols = ['smtp_host', 'smtp_port', 'smtp_secure', 'smtp_user', 'smtp_pass', 'from_email', 'from_name'];
+  for (const name of cols) {
+    // eslint-disable-next-line no-await-in-loop
+    const has = await knex.schema.hasColumn('mail_accounts', name);
+    // eslint-disable-next-line no-await-in-loop
+    if (has) await knex.schema.alterTable('mail_accounts', (t) => t.dropColumn(name));
+  }
+};