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>

internal/service/metatube.go

@@ -273,7 +273,7 @@ func (p *MetaTubeProvider) applyAuthHeader(req *http.Request, token string) {
 		req.Header.Set("X-Token", token)
 	}
 	req.Header.Set("Accept", "application/json")
-	req.Header.Set("User-Agent", "MMTL/1.0 (MetaTube Client)")
+	req.Header.Set("User-Agent", "MeBox/1.0 (MetaTube Client)")
 }
 
 func (p *MetaTubeProvider) convertSearchResultToMatch(query string, res *MetaTubeSearchResult) *Match {