Move system design forward and secure local setup

[xymeow]

What changed

• move System design / 系统设计 immediately after How to use / 如何使用
• align the bilingual Quick start into Daily start, First installation, and Upgrade
• keep routine startup free of clone, pull, and flash steps
• link troubleshooting and hardware recovery directly to the published Wiki pages
• align the bilingual QR and debugger-free installation guidance
• default the watcher admin setup service to 127.0.0.1
• advertise concrete bind hosts accurately and discover a LAN URL only for explicit 0.0.0.0
• reject unsupported IPv6 bind literals early with a clear configuration error

Why

The system diagram explains the relationship between the phone, watcher, mod, and optional TTS and belongs near the operating flow, before installation details. The setup admin service was documented and configured in the example as loopback-only, but the omitted-field code fallback still exposed it on all IPv4 interfaces.

User and developer impact

• readers see how the system works immediately after learning how to use it
• routine match-day startup no longer looks like an upgrade procedure
• the optional watcher admin page is local-only unless 0.0.0.0 is explicitly configured on a trusted LAN
• the primary device-hosted phone setup flow is unchanged
• no match parsing, commentary, delivery, TTS, mod, or host behavior changes

Validation

• python3 -m unittest discover -s tools -p 'test_*.py' — 168 passed
• focused setup-server tests — 6 passed
• node tools/test_stackchan_mod_web_behavior.mjs — passed
• all local Markdown/image links resolve
• git diff --check — passed