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>

.github/ISSUE_TEMPLATE/config.yml

@@ -1,5 +1,5 @@
 blank_issues_enabled: true
 contact_links:
-  - name: Telegram MMTL 交流群
-    url: https://t.me/MMTL
+  - name: Telegram MeBox 交流群
+    url: https://t.me/MeBox
     about: 适合快速交流部署经验、使用问题和排查线索。

.github/ISSUE_TEMPLATE/feature_request.md

@@ -12,7 +12,7 @@ assignees: ""
 
 ## 期望方案
 
-请描述你希望 MMTL 如何工作。
+请描述你希望 MeBox 如何工作。
 
 ## 使用场景