feat(ui): flat nav rail, compact assistant, responsive layouts

Implements the redesign from docs/ui-audit.html. Rearrangement only — no
feature was removed; every control that moved kept its handler, and the
gates that used to hide things now grey them out instead.

Navigation
  * The rail is one flat list: the five Workspace sub-views sit at the top
    level instead of behind an accordion, with Dashboard/Monitoring pinned
    at the foot and Settings below them.
  * Cowork and GraphRAG stay listed and greyed while no project is
    selected, rather than vanishing and resizing the menu under the user.
  * Monitoring keeps its eight sub-views in its own tab strip (unhidden)
    instead of doubling the rail's length.
  * _goto now moves the highlight itself, fixing a long-standing bug where
    programmatic navigation left the rail pointing at the previous screen.
  * Rail header gained the project picker and "New chat"; RECENTS lists the
    active project's threads. Both are second views of existing state — the
    Cowork toolbar button and the full History panel are untouched.
  * Provider / language / theme moved from the top bar to an account row at
    the foot of the rail (same widgets, same signals).

Screens
  * Co4E: the flow tab strip is gone (per the design); Flow Status became a
    toolbar toggle with its own way back, and the three icon-only tabs became
    four labelled, foldable sections in one column. One flow open at a time
    is the one capability this costs; background runs are unaffected.
  * Dashboard: header split into two rows; cost promoted to a hero card.
  * Monitoring Overview: one scrolling column of titled sections; the model
    price table got its own full-width section instead of sharing a row with
    the CPU meters.
  * Settings and Task editor gained a section index down the left.
  * Help dock: 84x64 launcher + chevron became one 26px dot that expands to
    a labelled pill on hover; "hide to the edge" moved into the panel's menu.

Layout
  * The window's minimum width dropped from 1453px to 768px. The main cause
    was a QTabWidget taking its minimum from the widest page even when that
    page is hidden, so Co4E was forcing Project and Cowork wide.
  * Secondary panes fold themselves on a narrow window and restore when it
    grows, never overriding a fold the user made.
  * The long dialogs no longer scroll sideways at any font size.

Verification: tools/check_*.py build a real MainWindow offscreen against a
copy of ~/.cowork_local with the schedulers no-oped. check_design_parity.py
reads its checklist straight from the audit page's own proposals.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/capture_screens.py

@@ -132,48 +132,20 @@ def main() -> int:
     nav_state = {"label": "", "expected": ""}
 
     def nav_to(win, page: int, sub=None, expect: str = "") -> None:
-        """Navigate the way a user does: SELECT the nav-rail row.
+        """Navigate the way a user does, and record where the rail ends up.
 
-        Calling ``win._goto()`` directly swaps the content but leaves the rail
-        highlighting whatever was selected before — so a Co4E screenshot showed
-        the content of Co4E with "Workspace" still lit. Setting the current item
-        fires currentItemChanged → _navigate → _goto, i.e. both halves.
+        Since the rail became a flat list, ``_goto`` moves the highlight itself
+        (``_select_nav_row``), so this no longer needs the two-step workaround
+        that existed while selecting a Workspace child destroyed the row being
+        selected.
         """
-        win._ensure_page(page)                    # build lazy page + its children
-        item = win._nav_items[page]
-        if sub is None:
-            win.nav.setCurrentItem(item)
-            app.processEvents()
-        else:
-            # Two steps, because selecting the row alone does not survive.
-            #
-            # Navigating to Workspace runs refresh(), which emits
-            # subtabs_changed → _reload_nav_children → takeChildren(); that
-            # DESTROYS the row just selected and the highlight falls back to the
-            # parent. Retrying only re-triggers the same cascade. (Real app bug,
-            # reproduced in test_nav_bug.py: 5/5 Workspace rows lose the
-            # highlight, 0/8 Monitoring rows do.)
-            #
-            # So: drive the content first, let the rebuild settle, then set the
-            # highlight with signals blocked so it cannot cascade again. The
-            # screenshot then shows what the user *should* see; the underlying
-            # bug is reported separately in the audit page.
-            win._goto(page, sub)
-            app.processEvents()
-            app.processEvents()
-            if item.childCount() == 0:
-                win._reload_nav_children(page)
-            item.setExpanded(True)
-            target = next(
-                (item.child(i) for i in range(item.childCount())
-                 if (item.child(i).data(0, Qt.UserRole) or {}).get("sub") == sub),
-                None)
-            assert target is not None, f"nav child page={page} sub={sub} not found"
-            blocked = win.nav.blockSignals(True)
-            win.nav.setCurrentItem(target)
-            win.nav.blockSignals(blocked)
+        win._ensure_page(page)
+        win._goto(page, sub)
         app.processEvents()
-        cur = win.nav.currentItem()
+        app.processEvents()
+        cur = next((t.currentItem() for t in (win.nav, win.nav_bottom)
+                    if t.currentItem() is not None and t.currentItem().isSelected()),
+                   None)
         nav_state["label"] = cur.text(0) if cur is not None else ""
         nav_state["expected"] = expect or nav_state["label"]