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/nginx.conf

@@ -184,7 +184,11 @@ server {
     # meta tags never reach them. Route those UAs to backend's /og handler
     # via internal rewrite; humans fall through to the SPA via try_files.
     location ~ ^/gallery/(?<gallery_slug>[A-Za-z0-9_-]+)(?:/[^/]+)?/?$ {
-        if ($http_user_agent ~* "(facebookexternalhit|facebot|Twitterbot|WhatsApp|Slackbot|TelegramBot|SkypeUriPreview|Discordbot|LinkedInBot|Pinterest|vkShare|redditbot|Embedly|iframely|Snapchat|Applebot|Mastodon|Bluesky|OpenGraph)") {
+        # Keep this list in sync with SOCIAL_CRAWLER_PATTERNS in
+        # backend/src/services/galleryOgService.js. WhatsAppBot / wa-bot
+        # and LinkPreview / Slack-ImgProxy added in #521 to catch
+        # business-API preview fetchers that aren't the main WhatsApp app.
+        if ($http_user_agent ~* "(facebookexternalhit|facebot|Twitterbot|WhatsApp|WhatsAppBot|wa-bot|Slackbot|Slack-ImgProxy|TelegramBot|SkypeUriPreview|Discordbot|LinkedInBot|Pinterest|vkShare|redditbot|Embedly|iframely|Snapchat|Applebot|Mastodon|Bluesky|OpenGraph|LinkPreview)") {
             rewrite ^ /og/gallery/$gallery_slug last;
         }
         try_files $uri $uri/ /index.html;