fix(app): chat UI polish — persistent dot, rail alignment, cohesive segments, smooth scroll

[jeonghun-jj-lee]

Closes #628

Summary

Implements the four chat UI polish improvements for a seamless messaging feel:

1. Cohesive text segmentation (chunk-boundaries.ts)

• Settled text parts render as multiple bordered cards at natural boundaries
• Label heuristic: short labels (<40 chars ending with :) stay with their content
• List-to-list guard: consecutive list blocks never split
• Heading coherence: a heading at the start of content stays with its body
• Monotonic: boundaries only grow during streaming (never retract)
• 22 tests covering all edge cases

2. Smooth scroll (smooth-scroll.ts)

• Custom RAF-based 180ms ease-out replaces instant scrollToEnd()
• Cancels immediately on user scroll gesture
• prefers-reduced-motion falls back to instant
• 6 tests with mocked RAF for deterministic verification

3. Persistent dot overlay (thought-rail.tsx)

• Turn-spanning sticky overlay — the AmicoWave never remounts while running
• CSS position: sticky; top: 12px keeps it visible during long streams
• 250ms spring animation (cubic-bezier(0.34, 1.56, 0.64, 1)) on step transitions
• Crossfade (150ms opacity) on turn completion; per-row done-dots appear underneath
• Deterministic vertical offsets by group type (no TreeWalker): 6 tests

4. Rail alignment

• md:pl-3 on TimelineRowFrame aligns the rail inside the composer's visible left boundary

Verification

• 1713 tests pass (119 test files, 0 failures)
• Extension build passes cleanly (esbuild)
• All new modules transpile and export correctly
• prefers-reduced-motion disables all animations (global rule + per-component)

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[jeonghun-jj-lee]

Fresh implementation on rebased branch

All changes are now committed to the opencode fork at local/amicode (commit 4e17c126a9):

What's in:

• Label heuristic in message-part-text.ts — short labels (<40 chars + colon) stay with content
• CSS empty-fragment safety net — [data-prose-fragment]:empty { display: none }
• Smooth scroll utility — 180ms ease-out RAF loop, cancels on gesture, reduced-motion fallback
• Smooth scroll wired into virtualizer's scrollToFn (only for small anchor-to-bottom increments)
• Rail alignment — md:pl-3 on assistant row frames
• Persistent overlay dot — turn-spanning sticky HarmonicDot, positioned from virtualizer measurements
• Per-row done-dot only — running dot removed from ThoughtRail, fragment TreeWalker removed
• 250ms spring transition — cubic-bezier(0.34, 1.56, 0.64, 1) on dot + rail line
• Crossfade class for turn completion

Test results:

• 15/15 chunk-boundary tests pass (session-ui)
• 11/11 timeline tests pass (app: thought-rail + smooth-scroll)
• Full binary build + smoke test passes
• All 30 packages typecheck clean