feat(品牌): 支持 _local 外部优先 Logo 与图标覆盖

新增 /admin/brand-assets 静态挂载，并在前端与桌面端统一采用外部优先、内置兜底的品牌资源解析；同时补充 BRANDING 文档与 README 入口说明。

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/BRANDING.md

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+# Branding Guide
+
+This project supports **external-first branding**.  
+Put your branding assets under `_local/branding`, and Oclaw will use them first.  
+If an external file is missing, Oclaw falls back to built-in defaults.
+
+## Directory
+
+Create this folder in the repository root:
+
+`_local/branding/`
+
+## Supported Files
+
+- `logo.svg`
+  - Used by admin sidebar logo and chat assistant logo/avatar.
+  - URL path: `/admin/brand-assets/logo.svg`
+- `desktop.ico` (recommended on Windows)
+  - Preferred desktop window icon for Electron.
+- `logo.png` (desktop fallback)
+  - Used as desktop icon fallback when `desktop.ico` is absent.
+
+## Resolution Order
+
+### Web UI (Admin + Chat)
+
+1. `_local/branding/logo.svg`
+2. Built-in fallback: `interfaces/admin/static/oliver.svg`
+
+### Desktop (Electron window icon)
+
+1. `_local/branding/desktop.ico`
+2. `_local/branding/logo.png`
+3. `_local/branding/logo.svg`
+4. Built-in fallback: `interfaces/admin/static/oliver.svg`
+
+## How to Apply
+
+1. Put your files into `_local/branding/`.
+2. Restart gateway (and desktop app if using Electron).
+3. Hard refresh browser page if needed.
+
+## Notes
+
+- `/admin/brand-assets/*` is served with no-cache headers to reduce stale asset issues.
+- `_local/*` is git-ignored by default, so tenant/customer branding stays local.