Feature: Limit initial timeline load window

[mjkatgithub]

Goal

Reduce perceived startup latency and scroll depth by limiting the initial timeline
window and loading messages incrementally via scrolling.

Scope

• Cap initial timeline window to a defined message budget
• Replace button-based loading with incremental scroll loading in both directions
• Preserve timeline continuity and ordering
• Avoid regressions for reaction/reply/thread rendering in loaded chunks
• Keep loading behavior resource-friendly (in-flight guards and cooldown)

Branch

feature/timeline-window-budget

Acceptance Criteria

• Initial chat open renders with reduced message volume
• Messages load incrementally via scrolling (up/down)
• Message order and grouping remain correct across pagination/window shifts
• User can still reach older history by repeated scrolling
• Last-read message is positioned approximately in the viewport center on load
• If there are no newer unread messages, initial viewport starts at bottom

Test Checklist

Unit

• Timeline-window mapping and merge behavior
• Pagination cursor/state updates

Integration

• Chat state updates correctly when older messages are appended
• Existing message UI features keep working after pagination/window shifts

E2E

• Open seeded room and verify smaller initial history window
• Scroll up/down multiple times and verify continuity