fix(frontend): skip short browser resumes and soften message refreshes

[hmans]

What changed

• Skip may-have-missed-messages refreshes for short browser visibility/pageshow resumes when the tab was hidden for less than 30 seconds.
• Add coverage for short and long browser resume paths, including the pageshow case.
• Change room message refreshes to use a larger soft-refresh window so the latest timeline can be reloaded without collapsing already-loaded history.
• Introduce explicit cursor modes for snapshot replacement so refreshes can either replace, preserve, or advance the visible window depending on context.
• Add regression coverage for soft-refreshing the latest room window while keeping older loaded messages intact.

Why

• Short tab switches and brief browser resumes were triggering unnecessary refresh work.
• Refreshing the latest room window with a small page size could drop already-loaded history from the UI, causing the timeline to visually jump or collapse.
• The new cursor handling keeps refresh behavior aligned with the intent of each refresh path and preserves the existing message buffer where appropriate.