Wire maintenance page into deploy script as a dynamic toggle

Replaces the earlier auto-fallback-on-upstream-error approach with an
explicit flag-file toggle controlled by the deploy script. The flag
is touched before stopping the app and removed on successful finish
(or via trap if the deploy aborts), so a failed deploy doesn't strand
the site in maintenance.

- nginx/goodwalk.co.nz.svelte.conf.example: error_page 503 routes to
  /maintenance.html (internal); /m/ serves static maintenance assets;
  the / and /api/submit blocks return 503 when /etc/nginx/conf.d/
  maintenance.flag exists.
- nginx/maintenance.html: brand-styled "Be right back" page — full
  Goodwalk green background, white card with yellow accent, real
  Goodwalk logo, contact details fallback, auto-reload after 60s.
- nginx/logo.png: maintenance-time logo (served from /m/logo.png).
- nginx/nginx.conf: reverted the earlier auto-fallback edits; this
  file is not deployed (the prod conf is goodwalk.co.nz.svelte.conf
  .example).
- scripts/deploy-remote.sh: copies maintenance.html + logo into the
  nginx container, reloads nginx so the new conf is live, touches
  the flag, then runs the rebuild, then clears the flag. Adds a
  trap-based clear_maintenance_flag fallback. Also adds a defensive
  env-file merger that appends new keys from deploy.env.template
  without clobbering live values, with a timestamped .env backup.

Plus a small a11y polish unrelated to maintenance:
- ServicesSection: "Learn more" links now include screen-reader-only
  "about <Service>" context.
- base.css: adds .visually-hidden utility class.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/styles/base.css

@@ -34,3 +34,15 @@ input,
 textarea {
   font: inherit;
 }
+
+.visually-hidden {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+  white-space: nowrap;
+  border: 0;
+}