Rebrand MMTL to MeBox (name, logo, Docker image) (#17)

* Rebrand MMTL to MeBox across codebase and assets

Rename the project display name, Go module path, environment variable
prefix (MEBOX_*), Docker image references, and UI branding from MMTL/mmtl
to MeBox/mebox. Replace logo assets with the new MeBox icon and keep
legacy SQLite migration support for existing mmtl.db deployments.

Co-authored-by: truewhile <truewhile@users.noreply.github.com>

* Fix logo icons: use cube-only crop without truncated text

Previous icon generation cropped too much of the source image, including
partial MeBox wordmark text that was cut off in square icon containers.
Regenerate logo-64/192/512, favicon, and SVG from cube-only region.

Co-authored-by: truewhile <truewhile@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

internal/config/config.go

@@ -4,7 +4,7 @@
 //  1. 内置默认值
 //  2. 工作目录中的 config.yaml（嵌套格式）
 //  3. config/*.yaml 分片文件（按模块）
-//  4. 以 MMTL_ 为前缀的环境变量
+//  4. 以 MEBOX_ 为前缀的环境变量
 package config
 
 import (
@@ -17,7 +17,7 @@ import (
 )
 
 // EnvPrefix 是所有环境变量驱动的覆盖使用的前缀。
-const EnvPrefix = "MMTL"
+const EnvPrefix = "MeBox"
 
 // Load 从默认值 / 文件 / 环境读取配置。
 //