fix(og): brandable static title + wider crawler UA coverage (#521)

@Rekoo-PS reported that gallery URLs sent via the WhatsApp Business
API render an unbranded "PicPeak - Photo Sharing Platform" preview
even though manual link sends from the WhatsApp app pick up the
per-event rich preview correctly. Two root causes, two fixes:

1. WhatsApp Business and 3rd-party preview services (Twilio,
   LinkPreview.net, etc.) don't always crawl with the recognisable
   "WhatsApp/X.Y.Z" UA we matched in nginx + galleryOgService.
   Extend the regex (both copies) to also catch WhatsAppBot, wa-bot,
   LinkPreview, and Slack-ImgProxy.

2. Even with broader UA coverage, some senders cache metadata with
   no UA at all and fetch the static SPA shell. That shell's
   <title> was hard-coded to "PicPeak - Photo Sharing Platform" —
   embarrassingly generic for any self-hosted brand. Switch to
   Vite's %VITE_DEFAULT_TITLE% / %VITE_DEFAULT_DESCRIPTION% HTML
   substitution so self-hosters can bake their brand into the
   fallback at build time. Defaults stay "PicPeak" so the upstream
   image doesn't change behaviour for anyone.

The per-event rich preview path (handleGalleryOgRequest, fired on
matched crawler UAs) is unchanged — this only improves the fallback
for unrecognised UAs and for the SPA-shell title that humans see in
their browser tab.

Adds a vite.config plugin to provide the defaults when env vars
aren't set, so unsubstituted "%VITE_..." literals never reach the
built HTML. Adds .env.example entries explaining the override.

Tests: extend galleryOgService.shareImage.test.js with an
isSocialCrawler suite that pins every documented UA (incl. the new
ones) plus three browser UAs (negative) and null/empty edge cases.
Verified locally: `vite build` with VITE_DEFAULT_TITLE="MyBrand"
produces <title>MyBrand</title> + og:title="MyBrand"; without the
env var falls back to "PicPeak".

Refs: #521

frontend/.env.example

@@ -1,3 +1,19 @@
+# Static HTML fallback for social-link previews (#521).
+#
+# Most link previews (WhatsApp, Facebook, Slack, etc.) hit the backend's
+# per-event OG endpoint and get the actual event name + branding. Some
+# third-party preview services and the WhatsApp Business API cache
+# metadata with a non-crawler User-Agent and end up reading these static
+# values instead. Set these to your brand so that fallback isn't generic
+# "PicPeak - Photo Sharing Platform".
+#
+# These are baked into index.html at build time, so they take effect on
+# the next `npm run build` / docker build. Live admin Branding settings
+# do NOT propagate here — for that, use the per-event OG endpoint, which
+# always serves the live branded preview.
+VITE_DEFAULT_TITLE=PicPeak
+VITE_DEFAULT_DESCRIPTION=Photo gallery shared with PicPeak.
+
 # Backend API URL
 # For local development with Docker:
 VITE_API_URL=http://localhost:3001/api