fix: minor UI polish

[brightyorcerf]

Linked issue

• Closes [Tracking] Minor UI Polish Follow-ups #425
• Related fix: correct spelling errors in UI text #374

Existing related work reviewed

• Issues/PRs reviewed: fix: correct spelling errors in UI text #374

Overlap assessment

• Classification: Enhancement / Branding Update
• Overlapping items: None
• Why this is not duplicate/superseded: This specifically targets the v5.5 transition to the MUIOGO identity, which consolidates several minor UI fixes that were previously unaddressed.

Why this PR should proceed

• Resolving UI inconsistencies is critical for the professionalization of the v5.5 release. Improving navigation discoverability and ensuring LaTeX (MathJax) responsiveness directly enhances the user experience for researchers using diagnostic views.

Summary

What changed:

• Updated copyright metadata to 2026 and restored the branded favicon in index.html.
• Standardized CSS button classes to btn-white and resolved duplicate class declarations.
• Fixed multiple typos ("Please", "Deleting") and standardized loader ellipsis convention.
• Added "Back" navigation controls to DataFile and ModelFile pages.
• Implemented horizontal scroll containers for MathJax equations in ModelFile.html.
• Migrated all user-facing labels from 'MUIO' to 'MUIOGO' globally.

Why:

• To ensure a cohesive brand identity under the new MUIOGO name and to fix layout/typographical regressions that affected the UI's polish and accessibility

Validation

• Tests added/updated (or not applicable)
• Validation steps documented
• Evidence attached (logs/screenshots/output as relevant)

Documentation

• Docs updated in this PR (or not applicable)
• Any setup/workflow changes reflected in repo docs

Scope check

• No unrelated refactors
• Implemented from a feature branch
• Change is deliverable without upstream OSeMOSYS/MUIO dependency
• Base repo/branch is EAPD-DRB/MUIOGO:main (not upstream)

Exception rationale

blank

[brightyorcerf]

I've bundled the specific UI fixes identified by the community in #425.
Hopefully, this centralizes the 'Polish' track into a single review to save on triage/merge overhead as suggested.