fix(app): restore timeline scroll position

[Brendonovich]

Summary

• persist each session timeline position through the existing debounced layout scroll store
• restore the exact visible virtual row and viewport offset across titlebar tab switches and reloads
• load older history when newer messages shift the latest page
• cancel restoration on user input and recover cleanly from history loading failures
• preserve message-hash navigation and latest-message bottom anchoring precedence
• add smoke coverage for exact row restoration across tabs and after appending 240 messages

Validation

• bun typecheck from packages/app
• focused layout-scroll and hash-scroll unit tests: 6 passed
• exact-row titlebar switch and shifted-history reload smoke test passed
• production timeline streaming benchmark passed with no geometry drift or remounts

Benchmark

• baseline completion: 60.7s; long-task time: 14.0s
• latest completion: 57.0s; long-task time: 10.0s
• machine-dependent measurements; no regression observed

[github-actions]

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