Squash merge of PR #717 — rebased on behalf of @franksong2702.
## What it does
Fixes#680. Footer chrome (timestamps, copy, edit, regenerate) is now hover-only for both user and assistant message rows, consistent throughout the conversation. The last assistant turn keeps cumulative usage visible at rest; timestamp and actions are revealed inline on hover in the same row.
Key changes:
- `static/ui.js`: new `_formatMessageFooterTimestamp()` (local timezone, cross-day fuller format); `timeHtml` no longer gated to user-only; last assistant usage moved from separate `.msg-usage` div to inline `.msg-usage-inline` span in the footer
- `static/style.css`: `.msg-foot-with-usage` class + rules; assistant footer opacity changed from 0.45 to 0 (hover-only); `:focus-within` alongside `:hover` for keyboard users
- `api/streaming.py`: `_restore_reasoning_metadata()` now preserves `_ts`/`timestamp` for unchanged historical messages
- `tests/test_sprint49.py`: 8 new tests covering rendering contract, hover CSS, timestamp preservation
Tests: 1518 passed. QA: 20/20. Browser verified. Reviewed and approved by @nesquena and @aronprins.
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