fix(chat): restore per-session reading positions

[Zhangs-11]

Summary

• remember each conversation's visible reading anchor while Pi Web remains open
• restore the same paragraph offset when returning to that conversation, loading earlier history pages when necessary
• keep tail-following conversations at the bottom and let explicit search-result navigation override remembered positions

The memory is page-local: refreshing or reopening Pi Web continues to start an existing conversation at the bottom.

Demo

Recorded from commit 3f9c2c1706e1175f13da93449dae5755b20e520b on the PR branch, served from that tree at http://127.0.0.1:30153 with next dev because this repository prohibits running next build during development. The run used a fresh isolated Pi agent directory, workspace, and browser profile with the normal application transport. Real GPT-6-Astra model rounds created both demonstrated conversations; no fixtures, mock transports, synthetic application events, or test-only hooks were used.

Validation

• npm test — 887 tests passed
• tsc --noEmit
• npm run lint
• git diff --check
• real browser QA: scroll a model-backed conversation to an older paragraph, switch conversations, return, and verify the same paragraph and pixel offset are restored
• real browser QA: a conversation left at the bottom returns to the bottom; a full page reload clears the in-memory position