feat(chat): add floor navigation rail with bookmarks to the transcript

[24baigei]

Add a floor navigation rail on the right edge of the chat transcript for quickly locating user messages in long conversations.

• Collapsed state shows a bare column of dashes (height-adaptive count via ResizeObserver, evenly sampled with bookmarked floors always kept); the current floor maps to the nearest sampled marker so the column never jitters while scrolling. Hovering expands a glass panel listing each user message's first characters; clicking jumps to that message.
• Only user-sent messages count as floors: floors derive from timeline items with kind === "user", so tool calls/results (folded into assistant groups) and system prompts are excluded by construction.
• Jumping drives virtualizer.scrollToIndex with a multi-frame settle loop that converges under dynamic row measurement and is cancelled by user wheel/touch/key input, a superseding jump, or unmount. A new ScrollFollowHandle.breakFollow() (historyKey semantics with measured hasOverflow) detaches bottom-follow first so the jump is not re-pinned; the web mirror of useScrollFollow.ts is updated byte-identically.
• The active floor tracks the viewport top edge (+8px) to match the jump alignment, with a bottom special case (scrollHeight coordinates) so the last floor wins when pinned at the bottom.
• Floors can be pinned: bookmarks persist per conversation by stable message id in a versioned localStorage blob (LRU-capped at 200 conversations, memory and disk trimmed together) and render as an amber pinned section at the top of the panel.
• The rail avoids the composer overlay (bottomReservePx), hides for <2 floors, and keeps keyboard access via focusable dash buttons.

Includes unit tests for floor derivation, preview truncation (code-point safe), sampling continuity, nearest-marker resolution, and bookmark persistence/eviction.