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/stores/auth.ts

@@ -56,7 +56,7 @@ export const useAuthStore = create<AuthState>()(
       },
     }),
     { 
-      name: 'mmtl-auth',
+      name: 'mebox-auth',
       partialize: (state) => ({ 
         token: state.token, 
         refreshToken: state.refreshToken, 

web/src/stores/playProfile.ts

@@ -22,7 +22,7 @@ export const usePlayProfileStore = create<PlayProfileState>()(
         })),
     }),
     {
-      name: 'mmtl-play-profile',
+      name: 'mebox-play-profile',
       storage: createJSONStorage(() => sessionStorage),
     },
   ),