docs: update user manual to v3.12.3 with new features coverage (en/zh/ja)

Add documentation for features introduced since v3.12.0:

New docs:
- 3.4 Session Manager: browse, search, resume, delete sessions
- 3.5 Workspace & Daily Memory: OpenClaw workspace file editing

Updated docs:
- Add Lightweight Mode to interface overview and FAQ
- Add tray submenu structure (providers grouped by app)
- Add API Format selection (Anthropic/OpenAI Chat/OpenAI Responses)
- Add Auto-Fetch Models button documentation
- Add Claude Common Config quick toggles
- Add Codex 1M Context Window toggle
- Add Skill backup/restore lifecycle
- Expand Backup Management panel documentation
- Update WebDAV sync to v2 protocol with dual-layer versioning
- Add OpenCode/OpenClaw to quickstart activation table
- Update README version to v3.12.3

All changes synced across en, zh, and ja locales.

docs/user-manual/en/5-faq/5.1-config-files.md

@@ -12,12 +12,11 @@ Customizable location in settings (for cloud sync).
 
 ```
 ~/.cc-switch/
-├── cc-switch.db      # SQLite database
+├── cc-switch.db      # SQLite database (SSOT)
 ├── settings.json     # Device-level settings
-└── backups/          # Automatic backups
-    ├── backup-20251230-120000.json
-    ├── backup-20251229-180000.json
-    └── ...
+├── skills/           # Skill SSOT directory
+├── skill-backups/    # Skill backups (created on uninstall)
+└── db_backup_*.db    # Database backups
 ```
 
 ### Database Contents

docs/user-manual/en/5-faq/5.2-questions.md

@@ -183,6 +183,16 @@ chmod +x CC-Switch-*.AppImage
 2. Manually download and install the latest version
 3. If using Homebrew: `brew upgrade --cask cc-switch`
 
+## Lightweight Mode
+
+### How to Enter Lightweight Mode?
+
+Toggle "Lightweight Mode" from the system tray menu. The main window closes, and CC Switch runs as a tray-only app. Toggle again or click "Open main window" to exit.
+
+### App Uses Less Memory in Lightweight Mode?
+
+Yes. Lightweight Mode destroys the main window and its web view, reducing memory usage significantly while keeping tray menu functionality available.
+
 ## Getting Help
 
 ### Submit an Issue