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_frontend_scroll.py

@@ -40,10 +40,15 @@ def _fn_body(js: str, name: str) -> str:
 
 def test_phase18_gate_is_gone() -> None:
     """The follow-the-bottom machinery (phase 18) is removed by owner
-    direction 2026-08-27: no band constant, no gate function, and no
-    document-scroller measurement left anywhere in app.js."""
+    direction 2026-08-27: no page-level band constant, no gate
+    function, and no document-scroller measurement left anywhere in
+    app.js. (The phase-43 window-level THINKING_NEAR_BOTTOM_PX /
+    isThinkingNearBottom pair is a different band — the thinking
+    window's, not the page's.)"""
     js = _js()
-    assert "NEAR_BOTTOM_PX" not in js, "the 200px band constant must be gone"
+    assert not re.search(r"(?<![A-Z_])NEAR_BOTTOM_PX\b", js), (
+        "the 200px page-level band constant must be gone"
+    )
     assert "isNearBottom" not in js, "the pinned-to-bottom gate must be gone"
     assert "window.scrollY" not in js, (
         "nothing in app.js measures the page scroll offset anymore"