Add STEGASOO_PORT env var, improve RPi setup output, channel key accordion

- Add STEGASOO_PORT environment variable support (default: 5000)
- Update .env.example with port and fix channel key format docs
- Move channel key generation to collapsible accordion in Generate page
- Improve RPi setup.sh output with HTTPS and channel key instructions
- Add rpi/BUILD_IMAGE.md workflow documentation

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/.env.example

@@ -5,9 +5,10 @@
 STEGASOO_AUTH_ENABLED=true
 STEGASOO_HTTPS_ENABLED=false
 STEGASOO_HOSTNAME=localhost
+STEGASOO_PORT=5000
 
-# Channel Key (256-bit hex for private channel isolation)
-# Generate with: python -c "import secrets; print(secrets.token_hex(32))"
+# Channel Key (format: XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX)
+# Generate with: stegasoo generate --channel-key
 # Leave empty for public mode
 STEGASOO_CHANNEL_KEY=