fix(brand-title): runtime substitution so GHCR-image users can override (#521 follow-up)

@Rekoo-PS confirmed the prior #521 fix landed on beta but reported the
preview still shows the default "PicPeak" title — their brand is
"arkan-studio". Root cause: that fix used Vite's build-time
%VITE_DEFAULT_TITLE% substitution. Self-hosters running the pre-built
ghcr.io/the-luap/picpeak/frontend image can't override at build time
without rebuilding, so they were stuck with whatever the upstream
build baked in.

Pivot to runtime substitution: the frontend container now reads
BRAND_TITLE / BRAND_DESCRIPTION env vars on startup and envsubsts
them into index.html. Change the values in .env, restart the frontend
service, done — no rebuild required.

Mechanics:
  - frontend/index.html: tokens are now ${BRAND_TITLE} /
    ${BRAND_DESCRIPTION} (shell expansion syntax, passes through Vite
    unchanged into the built dist).
  - frontend/Dockerfile: install gettext (provides envsubst), snapshot
    /usr/share/nginx/html/index.html → index.html.tpl at build, install
    docker-entrypoint.sh, wire ENTRYPOINT to it. The .tpl is the
    immutable source — every container start re-renders index.html
    from .tpl, so restarts pick up new env values cleanly (no
    accidental "first-boot env stuck forever" trap).
  - frontend/docker-entrypoint.sh: applies defaults if env unset,
    runs envsubst (locked to BRAND_TITLE + BRAND_DESCRIPTION
    explicitly so /assets/*.js template literals aren't touched if
    anyone ever extends substitution to the bundle), execs nginx.
  - frontend/vite.config.ts: drop the htmlTitleDefaults plugin — no
    longer needed since substitution is fully runtime.
  - frontend/.env.example + .env.production.example: drop the
    VITE_DEFAULT_* docs (the vars no longer have effect).
  - docker-compose.yml + docker-compose.production.yml: pass
    BRAND_TITLE / BRAND_DESCRIPTION env into the frontend service
    with sensible defaults so unconfigured installs work unchanged.
  - .env.example: add BRAND_TITLE / BRAND_DESCRIPTION with comment
    pointing at the social-preview use case.

Verified end-to-end against the built image:
  - BRAND_TITLE="Arkan Studio" BRAND_DESCRIPTION="Wedding photographs
    by Arkan Studio" → index.html serves <title>Arkan Studio</title>
    + og:title="Arkan Studio" + og:description correctly substituted.
  - .tpl preserves ${...} tokens so the next restart can re-substitute.
  - Bundle assets unaffected.
  - Defaults applied when env unset → <title>PicPeak</title>.

Docs PR in picpeak-docs describes the two new env vars under
"Social link preview fallback" in the environment-variables reference.

Refs: #521

frontend/.env.example

@@ -1,19 +1,3 @@
-# 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