Fix the timeline loading a room's entire history on open - #1373

Description

Opening a room paginated until it ran out of history, scrolling the view up as it went. Worst rooms burned 8-10s of CPU per open on a Pixel 3a.

The viewport-fill effect compared processedEvents.length against a value captured in the same render, so the guard was always true and the condition reduced to !hasRealScrollRoom || atTop. It re-runs on every length change, so staying near the top kept it firing. Pre-existing since March, triggered by #1363 leaving fewer rendered events per page.

Also in here:

• setAtBottom was only called from the scroll handler, so a resize (composer, keyboard) left it stale and killed auto-follow. Now recomputed from the ResizeObserver.
• Notification sound gates on window focus instead of tab visibility. A visible but unfocused window is now silent unless background notification sounds is on.
• Desktop reads window focus from onFocusChanged, since focusin/focusout miss app switches. Mobile and web unchanged.

Pixel 3a, release builds, CPU per room open:

Fixes #1371

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).