feat(admin): add About page and declare MIT license (v1.38.5)

[iscarelli]

What

Adds an Admin → About page (/admin/about, admin only) and, with it, the
license file the project never had.

The page shows:

• the installed version (read from current_version(), never hard-coded);
• what the system is for — cataloging and stock control of 3D-printing filaments;
• open project: source is public under the MIT license, linking LICENSE;
• source code: https://github.com/iscarelli/spool-control
• support: iscarelli@gmail.com

Why the LICENSE file

The repository was public but declared no license (GitHub showed "no license"),
so an About page claiming MIT would have pointed at nothing. This PR adds the
canonical MIT text (Copyright (c) 2026 Ismael Carelli) at the root and a
short License section in the README.

Version

1.38.5 (PATCH). 1.39.0 stays reserved for the scale front, so app work on
main ships as PATCH until that lands.

Verification

• ./run-tests.ps1 → 187 passed (183 existing + 4 new in tests/test_admin_about.py:
  200 for admin with e-mail and GitHub link in the body, version rendered, 403 for
  viewer, 302 → /login for anonymous).
• EN/ES translation key parity: no missing, no extra keys.
• Page rendered locally in PT, EN and ES with no untranslated strings.

🤖 Generated with Claude Code