CPLAT-9497: wire up changes action in session actions menu

[gavin-jeong]

JIRA: https://sendbird.atlassian.net/browse/CPLAT-9497

Summary

• handle the configurable changes action for single-session and bulk session actions menus
• populate the change map and diff viewport so the URL menu can render diff previews
• add regression tests for single and bulk session changes flows

Test plan

• go test ./internal/tui/...
• manually verify g:changes opens diff preview from session view

[jinsekim]

LGTM!