fix: redirect expired sessions to login on 401

[sergey3bv]

Summary

• Adds a global 401 handler in KeycastApi that redirects to login page when auth expires;
• Prevents raw "missing authentication" API errors from showing in admin/support pages while preserving existing non-401 error handling.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• Improving the project

Related Issue

• Closes fix(auth): redirect expired support-admin sessions to login #162

Testing

• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification completed

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved