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/vite.config.ts

@@ -1,13 +1,34 @@
 /// <reference types="vitest" />
 // @ts-nocheck
 
-import { defineConfig } from 'vite'
+import { defineConfig, loadEnv } from 'vite'
 import react from '@vitejs/plugin-react'
 import type { UserConfig as VitestUserConfig } from 'vitest/config'
 
+// Inject defaults for the %VITE_DEFAULT_TITLE% / %VITE_DEFAULT_DESCRIPTION%
+// placeholders in index.html when the env vars aren't set (#521). Without
+// this, Vite would leave the literal "%VITE_DEFAULT_TITLE%" string in the
+// built HTML, breaking the link-preview fallback we're trying to create.
+//
+// Self-hosters override by exporting the env vars at build time
+// (typical Docker build pattern: --build-arg VITE_DEFAULT_TITLE="My Brand").
+function htmlTitleDefaults(mode: string) {
+  const env = loadEnv(mode, process.cwd(), 'VITE_')
+  const title = env.VITE_DEFAULT_TITLE || 'PicPeak'
+  const description = env.VITE_DEFAULT_DESCRIPTION || 'Photo gallery shared with PicPeak.'
+  return {
+    name: 'html-title-defaults',
+    transformIndexHtml(html: string) {
+      return html
+        .replaceAll('%VITE_DEFAULT_TITLE%', title)
+        .replaceAll('%VITE_DEFAULT_DESCRIPTION%', description)
+    },
+  }
+}
+
 // https://vite.dev/config/
 const config: VitestUserConfig = {
-  plugins: [react()],
+  plugins: [react(), htmlTitleDefaults(process.env.NODE_ENV || 'production')],
   build: {
     rollupOptions: {
       output: {