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>

web/index.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta name="theme-color" content="#faf8f5" />
-    <meta name="description" content="MMTL — 个人家庭媒体中心" />
+    <meta name="description" content="MeBox — 个人家庭媒体中心" />
     <link rel="icon" type="image/png" href="/brand/logo-64.png" />
     <link rel="icon" type="image/x-icon" href="/favicon.ico" />
     <link rel="apple-touch-icon" href="/brand/logo-192.png" />
@@ -14,7 +14,7 @@
       href="https://fonts.googleapis.com/css2?family=Cabinet+Grotesk:wght@400;500;600;700;800&family=Geist:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;500;600&display=swap"
       rel="stylesheet"
     />
-    <title>MMTL</title>
+    <title>MeBox</title>
   </head>
   <body class="font-body antialiased">
     <div id="root"></div>