docs: add comprehensive installation & deployment guide

docs/04-installation.md covers all install scenarios:
- Fresh install (one-click script, curl pipe, manual)
- Updating an existing install (update.sh flow, backup/restore, rollback)
- Branch/tag installs via --ref / WR_REF (with auto tag-vs-branch detection)
- Service management (start.sh vs start.py, port differences, custom ports)
- Docker compose deployment
- Directory structure and what update.sh preserves vs updates
- .env configuration reference (SQLite/MySQL, ports, Redis, New-API)
- Troubleshooting (download fail, missing Go/Python, port conflicts,
  git pull conflicts, post-update rollback, log locations)
- install.sh/update.sh parameter cheat sheet

README: link to the new guide from Quick Start, fix default password
(admin123 -> admin123456), list 04-installation.md in project tree.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: WebRouter

README.md

@@ -115,6 +115,8 @@ Don't want to install anything? Try the live demo at **[demo.webrouter.tech](htt
 
 ### Install & Run
 
+> 📖 For detailed instructions (branch/tag installs, manual setup, troubleshooting), see **[docs/04-installation.md](docs/04-installation.md)**.
+
 **Linux / macOS:**
 
 ```bash
@@ -163,7 +165,7 @@ The script backs up user data, fetches the latest source, rebuilds `wr-proxy`, t
 
 ```bash
 open http://localhost:5050
-# Default login: admin / admin123
+# Default login: admin / admin123456
 ```
 
 ### Add Your First Provider
@@ -284,6 +286,10 @@ webrouter/
 │   ├── docker-compose.yml
 │   └── nginx.conf
 ├── docs/                   # Documentation
+│   ├── 01-requirements.md
+│   ├── 02-product-plan.md
+│   ├── 03-architecture.md
+│   └── 04-installation.md   # Detailed install/update guide
 ├── data/                   # Runtime data
 └── .env                    # Environment config (auto-generated)
 ```