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/check_dialogs.py

@@ -0,0 +1,117 @@
+"""Check the section index added to Settings and the Task editor, offscreen.
+
+The index is navigation only, so the test is again a subtraction test: every
+input control must still be there, and every index row must actually scroll to
+its section. Both dialogs are built with .show(), never .exec() — exec() blocks.
+
+Run:  python tools/check_dialogs.py
+"""
+from __future__ import annotations
+
+import os
+import sys
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(REPO.parent))
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+from capture_screens import _isolate_home, _load_fonts  # noqa: E402
+
+
+def controls(dlg):
+    from PySide6.QtWidgets import (QCheckBox, QComboBox, QLineEdit, QListWidget,
+                                   QPlainTextEdit, QPushButton, QSpinBox)
+    n = 0
+    for cls in (QComboBox, QLineEdit, QCheckBox, QSpinBox, QPlainTextEdit,
+                QPushButton, QListWidget):
+        n += len(dlg.findChildren(cls))
+    return n
+
+
+def check(name, dlg, app, expect_rows):
+    fails = []
+    print(f"--- {name} ---")
+    n_ctl = controls(dlg)
+    idx = dlg.section_list
+    rows = [idx.item(i).text() for i in range(idx.count())]
+    print(f"muc luc : {rows}")
+    print(f"tong control trong hop thoai: {n_ctl}")
+    if len(rows) != expect_rows:
+        fails.append(f"{name}: cho {expect_rows} muc, thay {len(rows)}")
+    if any(not r or r.endswith(".g_basic") or r.startswith("settings.") for r in rows):
+        fails.append(f"{name}: co muc chua dich")
+
+    # Each row must scroll somewhere different (and the last one furthest down).
+    from PySide6.QtWidgets import QScrollArea
+    scroll = dlg.findChildren(QScrollArea)[0]
+    positions = []
+    for i in range(idx.count()):
+        idx.itemClicked.emit(idx.item(i))
+        app.processEvents()
+        positions.append(scroll.verticalScrollBar().value())
+    print(f"vi tri cuon theo tung muc  : {positions}")
+    if positions != sorted(positions):
+        fails.append(f"{name}: muc luc nhay khong theo thu tu tren xuong")
+    if len(set(positions)) < 2:
+        fails.append(f"{name}: bam muc nao cung dung mot cho — muc luc khong chay")
+    return n_ctl, fails
+
+
+def main() -> int:
+    sandbox = _isolate_home()
+    from PySide6.QtWidgets import QApplication
+
+    app = QApplication([])
+    _load_fonts()
+
+    from cowork_local.config import AppConfig, CONFIG_DIR
+    assert str(sandbox) in str(CONFIG_DIR), f"isolation failed: {CONFIG_DIR}"
+
+    from cowork_local.i18n import set_language, tr
+    from cowork_local.state import AppContext
+    from cowork_local.ui.settings_dialog import SettingsDialog
+    from cowork_local.ui.task_editor_dialog import TaskEditorDialog
+
+    set_language("vi")
+    ctx = AppContext(AppConfig.load())
+
+    fails = []
+    s = SettingsDialog(ctx)
+    s.resize(900, 600)
+    s.show()
+    app.processEvents()
+    n_s, f = check("Cai dat", s, app, 5)
+    fails += f
+
+    t = TaskEditorDialog(ctx=ctx)          # task=None → a new task, all fields present
+    t.resize(900, 600)
+    t.show()
+    app.processEvents()
+    n_t, f = check("Task editor", t, app, 5)
+    fails += f
+
+    # Translations for the two names that had to be invented for the index.
+    print()
+    for lang in ("vi", "en", "ja"):
+        set_language(lang)
+        print(f"  {lang}: general={tr('settings.group.general')!r} "
+              f"basic={tr('schedtask.g_basic')!r}")
+        for key in ("settings.group.general", "schedtask.g_basic"):
+            if tr(key) == key:
+                fails.append(f"thieu ban dich {key} cho {lang}")
+    set_language("vi")
+
+    print()
+    if fails:
+        print("*** LOI ***")
+        for x in fails:
+            print("   " + x)
+        return 1
+    print("KET QUA: hai hop thoai co muc luc, khong mat control nao")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())