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>

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

.github/workflows/docker-publish.yml

@@ -4,7 +4,7 @@ on:
   workflow_dispatch:
     inputs:
       version:
-        description: 'Image tag to publish, for example MMTL-v0.0.32'
+        description: 'Image tag to publish, for example MeBox-v0.0.32'
         required: true
         type: string
       ref:
@@ -42,7 +42,7 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           # 自动使用当前仓库所有者
-          images: ghcr.io/${{ github.repository_owner }}/mmtl
+          images: ghcr.io/${{ github.repository_owner }}/mebox
           tags: |
             type=raw,value=latest
             type=raw,value=${{ env.RELEASE_VERSION }}