fix(app): rail continuity + inset + crossfade

[jeonghun-jj-lee]

Context

Part of #628 — chat UI polish.
Blocked by #630 (persistent dot overlay must exist before this slice can land).

This slice adds the visual finishing layer on top of the overlay structure: a continuous rail line for the active turn, left inset alignment, and a crossfade transition when the turn completes.

Acceptance Criteria

• The rail line for the active turn is continuous (no per-row seams/gaps visible during streaming)
• The rail left edge sits inside the composer visible left boundary at md+ breakpoints
• On turn completion, the overlay dot crossfades out (150ms opacity) and the per-row done-dot appears underneath
• Rail line + dot spring use identical timing (250ms, same bezier) so they move in lockstep
• prefers-reduced-motion disables crossfade (instant swap)

Key Decisions

Decision	Rationale
Overlay draws the rail line for active turns	A single <span> from overlay top to bottom — no per-row segments needed while streaming, eliminates gap math
Per-row rail segments only for completed turns	Once a turn is done, it uses the existing static per-row lines (no overlay exists for it anymore)
md:pl-3 on TimelineRowFrame	Simplest fix — shifts content rightward at medium breakpoints, aligning rail gutter inside the composer interior
Crossfade via data-state="completed" + CSS transition	On turn completion, overlay gets attribute; CSS opacity: 0 over 150ms ease-out; after transitionend, remove overlay DOM
Rail and dot share timing (250ms same bezier)	Move in lockstep — the rail height extends to the new dot position at the same rate the dot slides

Testing Decisions

• Crossfade timing: overlay opacity reaches 0 within 150ms of turn completion (verify via transitionend event or computed style)
• Rail inset: at md+ viewport, rail line left position is > 0 relative to the compositor visible boundary
• No seams: during a streaming turn, only one continuous line element exists for the active turn (not N per-row segments)
• Lockstep: rail height transition and dot top transition use identical transition declaration

Constraints & Invariants

• Depends on slice 2 (fix(app): persistent dot overlay — turn-spanning sticky, step-boundary travel, deterministic offsets #630) overlay structure being merged
• Done-dots still render per-row (they become visible when the overlay fades)
• The crossfade transitionend cleanup must be resilient to rapid turn succession (if a new turn starts before fade completes, cancel the outgoing fade and destroy immediately)
• Rail line in the overlay uses the same LINE_X constant as per-row segments

Prior Art

• packages/app/src/pages/session/timeline/thought-rail.tsx — current per-row rail segments with negative-top bridging, LINE_X constant
• packages/app/src/index.css — current [data-slot="thought-rail-line"] transition (150ms ease-out)
• fix(app): persistent dot overlay — turn-spanning sticky, step-boundary travel, deterministic offsets #630 — the overlay structure this slice builds on