fix: remember and restore scroll position per session

[grunjol]

When leaving and re-entering a chat session, the scroll position is
now restored to where the user was, instead of always jumping to the
bottom (or top, since _scrollToBottom() fired before the ListView
was laid out).

Changes

_lastPixels — tracks the last known scroll position, updated in
_onScroll() on every scroll event. Storing in dispose() was
unreliable because hasClients is already false when the widget tree
tears down.

_savedPositions — a static map (sessionId → pixels) shared
across ChatScreen instances so returning to the same session
restores the previous scroll offset.

_fetchMessages() — if a saved position exists for the session,
uses WidgetsBinding.instance.addPostFrameCallback to jumpTo() the
saved offset after layout completes. On first visit, scrolls to the
bottom as before.

[rusty4444]

Hermes Agent Automated Review — PR #71

Verdict: APPROVE ✅

Summary

Remembers and restores scroll position per session. Saves last scroll position before dispose, restores it on session load with proper bounds clamping.

✅ Looks Good

• Per-session scroll position storage prevents losing reading context when switching sessions
• clamp(0.0, maxScrollExtent) prevents out-of-bounds scroll errors
• Falls back to scroll-to-bottom for new sessions (no saved position)
• Proper hasClients checks before accessing scroll controller

💡 Suggestions (non-blocking)

• The scroll restore logic is duplicated in _loadMessages and _sendMessage completion — consider extracting to a helper method _restoreScrollPosition()

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Reviewed by Hermes Agent — July 8, 2026