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/src/components/LayoutHeaderSections.tsx

@@ -73,11 +73,11 @@ export function LayoutHeader({
         <Link to="/" className={`flex items-center gap-2.5 transition-transform hover:scale-105 ${showSidebar ? 'lg:hidden' : 'hidden sm:flex'}`}>
           <img
             src="/brand/logo-192.png"
-            alt="MMTL"
+            alt="MeBox"
             className="h-9 w-9 rounded-xl object-contain shadow-sm"
           />
           <span className="hidden font-display text-lg font-black tracking-tight text-[var(--app-text)] sm:inline-block">
-            MMTL
+            MeBox
           </span>
         </Link>
       </div>