feat(scheduled): Send now button on scheduled/delayed messages

Adds a 'Send now' action to the scheduled-send view (both the toolbar and the
inline banner) so a queued message — whether explicitly scheduled or held by the
undo-send delay — can be sent immediately instead of only cancel/reschedule/edit.
Reuses the existing reschedule path (reschedules the submission to now), so no
new JMAP plumbing. Toolbar 'Cancel send' demoted to ghost so 'Send now' is the
single primary action. i18n added across locales.

locales/cs/common.json

@@ -565,7 +565,8 @@
       "add_to_contacts": "Přidat do kontaktů",
       "copied": "Zkopírováno!",
       "copy_failed": "Kopírování se nezdařilo"
-    }
+    },
+    "send_now": "Send now"
   },
   "email_composer": {
     "read_receipt_on": "Vyžádáno potvrzení o přečtení (kliknutím vypnete)",