feat: add custom Gitea landing page and re-enable public access

- Disable REQUIRE_SIGNIN_VIEW to allow public landing page
- Mount custom templates directory
- Add custom home.tmpl with simple welcome page and sign-in button
- Remove default Gitea promotional content

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

dev/compose.yaml

@@ -14,6 +14,7 @@ services:
       - /etc/timezone:/etc/timezone:ro
       - /etc/localtime:/etc/localtime:ro
       - ./public:/data/gitea/public:ro
+      - ./templates:/data/gitea/templates:ro
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
       USER_UID: 1000
@@ -37,7 +38,7 @@ services:
       GITEA__mailer__SMTP_PORT: 1025
       GITEA__mailer__FROM: ${EMAIL_FROM}
       GITEA__service__DISABLE_REGISTRATION: false
-      GITEA__service__REQUIRE_SIGNIN_VIEW: true
+      GITEA__service__REQUIRE_SIGNIN_VIEW: false
       GITEA__packages__ENABLED: true
       GITEA__actions__ENABLED: true
       GITEA__ui__THEMES: gitea-auto,gitea-light,gitea-dark,arc-green,edge-auto,edge-dark,edge-light,everforest-auto,everforest-dark,everforest-light,gruvbox-auto,gruvbox-dark,gruvbox-light,gruvbox-material-auto,gruvbox-material-dark,gruvbox-material-light,nord,palenight,soft-era,sonokai,sonokai-andromeda,sonokai-atlantis,sonokai-espresso,sonokai-maia,sonokai-shusia