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

backend/src/__tests__/galleryOgService.shareImage.test.js

@@ -35,6 +35,7 @@ const { getStorage } = require('../services/storage');
 const {
   buildOgMetadata,
   handleGalleryOgCover,
+  isSocialCrawler,
 } = require('../services/galleryOgService');
 
 // The service hits two tables in sequence:
@@ -237,3 +238,52 @@ describe('handleGalleryOgCover — 404 unless explicitly opted in', () => {
     expect(ensureThumbnail).not.toHaveBeenCalled();
   });
 });
+
+// Regression for #521 — WhatsApp Business API + 3rd-party preview
+// services use UAs that aren't "WhatsApp/X.Y.Z". If isSocialCrawler
+// misses them, those requests fall through to the static SPA shell
+// and the link preview ends up unbranded.
+describe('isSocialCrawler — extended bot coverage (#521)', () => {
+  it('matches every UA the README/changelog claims to support', () => {
+    // Pin the contract: each listed UA must hit the crawler path so the
+    // nginx rewrite + backend OG handler stay in sync. Adding a new UA
+    // here without also adding it to nginx.conf would silently regress.
+    const knownBots = [
+      // Main WhatsApp app
+      'WhatsApp/2.23.20.0',
+      // WhatsApp Business / Cloud API variants
+      'WhatsAppBot/1.0',
+      'wa-bot/2.0',
+      // Other messaging app crawlers
+      'facebookexternalhit/1.1',
+      'Twitterbot/1.0',
+      'Slackbot-LinkExpanding 1.0',
+      'TelegramBot (like TwitterBot)',
+      // 3rd-party preview services used by business-messaging stacks
+      'LinkPreview/1.0',
+      'Slack-ImgProxy/1.0',
+    ];
+    for (const ua of knownBots) {
+      expect(isSocialCrawler(ua)).toBe(true);
+    }
+  });
+
+  it('does not match a regular browser UA', () => {
+    const browsers = [
+      'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0 Safari/537.36',
+      'Mozilla/5.0 (iPhone; CPU iPhone OS 17_2 like Mac OS X) AppleWebKit/605.1.15',
+      // Browser UA that happens to contain "Mobile" — guard against an
+      // over-broad regex landing on it.
+      'Mozilla/5.0 (Linux; Android 14; Pixel 7) AppleWebKit/537.36 Chrome/120.0 Mobile Safari/537.36',
+    ];
+    for (const ua of browsers) {
+      expect(isSocialCrawler(ua)).toBe(false);
+    }
+  });
+
+  it('returns false for null/empty/undefined UAs', () => {
+    expect(isSocialCrawler(null)).toBe(false);
+    expect(isSocialCrawler(undefined)).toBe(false);
+    expect(isSocialCrawler('')).toBe(false);
+  });
+});