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>

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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

web/public/artwork-cache-sw.js

@@ -1,4 +1,4 @@
-const ARTWORK_CACHE_PREFIX = 'mmtl-artwork-'
+const ARTWORK_CACHE_PREFIX = 'mebox-artwork-'
 const ARTWORK_CACHE = `${ARTWORK_CACHE_PREFIX}v2`
 const MIN_CACHEABLE_ARTWORK_BYTES = 128
 const STRIP_QUERY_KEYS = ['token', 'profile_id', 'profile_pin_token']