Replace legacy ALPS license headers with MIT SPDX notices

[skilledwolf]

Summary

This PR aligns files that still refer to the historical ALPS Library or Application licenses with the repository-level MIT license.

Related discussion: #108

Changes

• Replace legacy ALPS license boilerplate with concise SPDX-License-Identifier: MIT notices.
• Remove obsolete preamble templates and scripts that generated the old headers.
• Remove the legacy license check from alps_inspect.
• Update user-visible license messages and documentation to reference MIT.
• Make the lattice-preview About dialog load the canonical LICENSE.txt rather than embedding another copy.

Why

The repository-level LICENSE.txt declares MIT, while a number of older source files, tools, and tutorials still named the superseded ALPS licenses and, in some cases, described different obligations. This PR removes that contradiction while keeping the canonical license text in one place.

Scope

Third-party files that already contain their own complete license notices are intentionally unchanged. Those notices may need to remain for attribution and license compliance.

Validation

• Confirmed that the targeted legacy ALPS license phrases no longer remain.
• Validated Python syntax and notebook JSON.
• Ran whitespace and staged-diff checks.

Follow-up

Maintainer confirmation in #108 is requested to verify that the affected per-file notices are historical rather than intentional licensing exceptions.

[skilledwolf]

Note: I cancelled the CI runs, since this PR doesn't functionally change anything (just comments).