Add carrier gateway field, calendar-only push, and menu preview to UI

Surfaces the settings/actions that only existed at the API or env-var
level: carrier_gateway was already an accepted setting but had no form
field; HA webhook and Gmail config were invisible from the UI entirely.
Adds a calendar-only push and a side-effect-free preview so the HA sync
and scraper can be tested without texting everyone. Secrets stay env-only
since the UI has no auth — only configured/not-configured status is
exposed.

Also fixes calendar_result reporting "ok" when HA_WEBHOOK_URL wasn't set
at all, and stops the calendar-only push from clobbering the SMS job's
last_run/last_result status fields.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KsJqAZ82Veyf5oihBr71S5

CLAUDE.md

@@ -26,6 +26,17 @@ Runs as one Docker container on the user's Unraid home server.
   automation (webhook trigger → `calendar.create_event`) turns this into an
   all-day calendar event with the menu. This makes the scrape here the
   single source of truth — HA doesn't poll or re-scrape anything.
+- **Manual actions (2026-09-03):** besides "Send Lunch Menu Now" (full job —
+  scrape, HA push, text everyone), the UI has "Push to Calendar Only"
+  (`POST /api/push-calendar` → `push_calendar_only()`, scrape + HA push, no
+  texts — for testing the calendar sync without spamming phones) and
+  "Preview Today's Menu" (`GET /api/preview` → `_scrape_menu()`, pure dry
+  run, no side effects at all). `carrier_gateway` is editable in the
+  Schedule card (dropdown of common US carriers + custom domain). Gmail
+  credentials and `HA_WEBHOOK_URL` stay env-only, not settings.json — the
+  UI has no auth, so secrets aren't made editable there; `GET
+  /api/integrations` just exposes whether each is configured (booleans,
+  not values) so the UI can show a badge.
 
 ## Local development