feat: configurable chat history scrollback (avoid losing older messages)

[amanthanvi]

Problem

In long-running threads, older chat history becomes inaccessible in the UI: as a conversation grows, the beginning of the thread disappears from the scrollback.

This makes it hard to recover context in longer chats and power-user sessions.

Current behavior

• The UI retains only a small window of conversation items per thread.
• As more items arrive (streaming deltas, tool output, etc.), earlier items are dropped locally.

Root cause (code)

The frontend unconditionally limits thread items via prepareThreadItems():

• src/utils/threadItems.ts has MAX_ITEMS_PER_THREAD = 200.
• prepareThreadItems() slices to the last 200 items (normalized.slice(-MAX_ITEMS_PER_THREAD)).
• Reducers call prepareThreadItems(...) during normal thread updates, so the cap applies continuously.

Proposal

Add an app-wide setting to configure thread history scrollback retention:

• Default stays 200 items (no behavior change for most users).
• Allow power users to increase to higher values (e.g. 500/1000/2000/5000).
• Optional Unlimited mode (stores all items returned by thread/resume + live items).

UX

Settings: Settings -> Display & Sound (or similar)

• Toggle: Unlimited chat history
• Number input: Max items per thread (disabled when Unlimited is on)
• Presets: 200 / 500 / 1000 / 2000 / 5000
• Help text warning about performance/memory costs for large values.

Note: existing thread context menu already has “Sync from server”; after increasing scrollback, users can sync to pull additional older history.

Performance considerations

Increasing retention can increase memory/DOM/render cost. To mitigate:

• Virtualize the message list (@tanstack/react-virtual is already in the repo and used elsewhere).
• Virtualize at the grouped-item level (tool groups + items), with ResizeObserver measurement like GitDiffViewer.

Acceptance criteria

• Default remains last-200 behavior.
• Setting persists and applies to new and existing threads.
• Higher limits retain more items; Unlimited retains all items received.
• No major regressions in scroll/auto-scroll behavior.
• Tests cover retention logic (default/custom/unlimited) and settings normalization.

[amanthanvi]

Follow-up PR: #427

This is a reduced-scope version of the earlier attempt (#423, closed): configurable scrollback setting + retention logic, but no conversation view virtualization to avoid the scroll-jump issue and keep complexity down.