feat(chat): thinking window scrolls again, follows the tail only while pinned

TODO.md L7: "Add scrolling back to the thinking block, but have it
autoscroll while thinking content is generating." Owner direction
2026-08-27 (roadmap A2) reverses the phase-21 no-scroll choice
(2026-08-24): details.thinking .thinking-text is user-scrollable again
(overflow-y: auto — the 320px clip stays, owner-confirmed), and the
phase-17 per-chunk bottom-pin is GATED: the window follows the live
tail only while the user is pinned near its bottom (THINKING_NEAR_
BOTTOM_PX = 32); scrolling up pauses the follow, returning to the
bottom re-arms it on the next chunk (the gate re-runs on every frame).

- frontend/assets/styles.css: .thinking-text overflow-y: hidden ->
  auto; the phase-21 owner-choice comment is replaced with the
  2026-08-27 direction; max-height: 320px and every other declaration
  in the rule byte-identical.
- frontend/assets/app.js: export const THINKING_NEAR_BOTTOM_PX = 32 +
  isThinkingNearBottom(textEl) (scrollHeight - scrollTop -
  clientHeight <= band); the thinking-handler pin becomes
  `if (block.open && isThinkingNearBottom(textEl))` — a scrolled-up
  reader is never re-pinned and a closed (restored) block is never
  pinned; everything else in the handler (and phase 42's no page
  scroll) untouched.
- tests/unit/test_thinking_scroll.py (new, replaces the deleted
  tests/unit/test_thinking_no_scroll.py): pins the CSS contract (auto
  + 320px + owner-direction comment, no hidden/scroll left), the
  exported 32px band, the gate math, the gated pin (no unconditional
  `if (block.open)` remains), and the surviving collapsed-restore pin.
- tests/e2e/test_thinking_scroll.py (new, mock-only, replaces the
  deleted tests/e2e/test_thinking_no_scroll.py — its pins asserted the
  reversed phase-21 behavior, so both phase-21 files are deleted in
  this commit): user scroll restored on the frozen 4s-hesitation tail
  (wheel is 1:1; click+Home keyboard — the plain div is not
  keyboard-focusable by design, tabindex is test scaffolding; the
  literal drag holds the user's position — headless Chromium's
  overlay scrollbars are not grabbable by synthetic mouse events,
  documented in the suite), follow-while-pinned at the 2nd-to-last
  and last chunk (±1px) with the last chunk's text inside the visible
  rectangle, no re-pin over ≥5 mid-stream chunks after a
  half-window scroll-up, re-pin on the next chunk after returning to
  the bottom, the CSS contract, plus the phase-11 (long answer: page
  scrolls, bubble overflow untouched) and phase-17 (restored
  collapsed block with full text) regressions.
- tests/unit/test_chat_persistence.py: the CSS pin flips with the
  contract (auto in, hidden out — owner direction 2026-08-27).
- tests/unit/test_frontend_scroll.py: the "page-level band constant is
  gone" pin now excludes the phase-43 window-level
  THINKING_NEAR_BOTTOM_PX (a different band — the window's, not the
  page's).

E2E (isolation): test_thinking_scroll 7/7 (twice); regressions
test_thinking_display 5/5, test_chat_persistence 4/4,
test_no_reply_autoscroll 5/5, test_smoke 3/3; unit+integration 725
passed, app/ coverage 99% (unchanged — frontend-only phase);
ruff + pyright clean.

tests/unit/test_chat_persistence.py

@@ -240,9 +240,9 @@ def test_restore_renders_collapsed_thinking_block() -> None:
 
 def test_thinking_block_css_uses_phase08_tokens() -> None:
     """Phase 17 styling (Phase-08 tokens, WCAG AA): the block frame, the
-    ≥44px summary control (brand-ink ≈8.7:1 on surface) and the live-tail
-    scratchpad (ink-soft ≈6.9:1 on surface, 320px cap; phase 21 removed the
-    user scroll — owner choice 2026-08-24)."""
+    ≥44px summary control (brand-ink ≈8.7:1 on surface) and the scratchpad
+    (ink-soft ≈6.9:1 on surface, 320px cap; user-scrollable again since
+    phase 43 — owner direction 2026-08-27, TODO.md L7)."""
     css = _css()
     block = re.search(r"details\.thinking \{([\s\S]*?)\n\}", css)
     assert block, "styles.css must style details.thinking"
@@ -262,6 +262,7 @@ def test_thinking_block_css_uses_phase08_tokens() -> None:
     tbody = text.group(1)
     assert "var(--ink-soft)" in tbody
     assert "max-height: 320px" in tbody
-    # Phase 21: no user scroll back — the window is a live tail only.
-    assert "overflow-y: hidden" in tbody
-    assert "overflow-y: auto" not in tbody
+    # Phase 43 (owner direction 2026-08-27): user-scrollable window again;
+    # the phase-17 bottom-pin (gated in app.js) is the autoscroll.
+    assert "overflow-y: auto" in tbody
+    assert "overflow-y: hidden" not in tbody