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/LayoutSidebarContent.tsx

@@ -73,7 +73,7 @@ function LayoutSidebarHeader({
       <Link to="/" className="flex items-center gap-3">
         <img
           src="/brand/logo-192.png"
-          alt="MMTL"
+          alt="MeBox"
           className="h-10 w-10 shrink-0 rounded-xl object-contain shadow-sm"
         />
         {sidebarExpanded && (
@@ -82,7 +82,7 @@ function LayoutSidebarHeader({
             animate={{ opacity: 1, x: 0 }}
             className="font-display text-lg font-extrabold tracking-tight text-[var(--app-text)]"
           >
-            MMTL
+            MeBox
           </motion.span>
         )}
       </Link>