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>

ui/settings_dialog.py

@@ -10,8 +10,8 @@ from PySide6.QtGui import QGuiApplication
 from PySide6.QtWidgets import (
     QCheckBox, QComboBox, QDialog, QDialogButtonBox, QFileDialog, QFormLayout,
     QGroupBox, QHBoxLayout, QLabel, QLineEdit, QListWidget, QListWidgetItem,
-    QMessageBox, QPushButton, QScrollArea, QSpinBox, QTreeWidget, QTreeWidgetItem,
-    QVBoxLayout, QWidget,
+    QMessageBox, QPushButton, QScrollArea, QSizePolicy, QSpinBox, QTreeWidget,
+    QTreeWidgetItem, QVBoxLayout, QWidget,
 )
 
 from ..config import PROVIDER_LABELS
@@ -42,6 +42,10 @@ class SettingsDialog(QDialog):
         outer = QVBoxLayout(self)
         scroll = QScrollArea()
         scroll.setWidgetResizable(True)
+        # Never sideways: the content must fit the width it is given and scroll
+        # only downwards. Anything too wide has to shrink (see _model_combo and
+        # _with_load), not push a second scrollbar onto the user.
+        scroll.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
         self._content = QWidget()
         root = QVBoxLayout(self._content)
 
@@ -59,6 +63,11 @@ class SettingsDialog(QDialog):
         self.notify_chk = QCheckBox(tr("settings.tray_notify"))
         self.notify_chk.setChecked(bool(data.get("tray", {}).get("notify_on_done", True)))
         top.addRow("", self.notify_chk)
+        # Zero-height anchor so the index can scroll to this section, which is a
+        # bare form rather than a group box.
+        self._anchor_general = QWidget()
+        self._anchor_general.setFixedHeight(0)
+        root.addWidget(self._anchor_general)
         root.addLayout(top)
 
         self._load_workers = []
@@ -291,10 +300,31 @@ class SettingsDialog(QDialog):
 
         note = QLabel(tr("settings.tip"))
         note.setObjectName("hint")
+        note.setWordWrap(True)      # otherwise this one line sets the dialog's width
         root.addWidget(note)
 
         scroll.setWidget(self._content)
-        outer.addWidget(scroll, 1)
+        # Five group boxes in one column, with no way to see what was further
+        # down — the index says what is in here and jumps straight to it.
+        from .widgets import section_index
+        self.section_list = section_index(scroll, [
+            (tr("settings.group.general"), self._anchor_general),
+            (tr("settings.group.provider"), prov_group),
+            (tr("settings.group.sandbox"), self.sandbox_group),
+            (tr("settings.group.parameter"), param_group),
+            (tr("routing.settings_group"), routing_group),
+        ])
+        body = QHBoxLayout()
+        body.setSpacing(10)
+        body.addWidget(self.section_list)
+        body.addWidget(scroll, 1)
+        outer.addLayout(body, 1)
+        # Floor the dialog at the width its own content needs AT THE CURRENT
+        # FONT. On a 125%/150% display everything is wider, and without this the
+        # form was simply cut off (or scrolled sideways) instead of the window
+        # refusing to get that small.
+        self.setMinimumWidth(self.section_list.width()
+                             + self._content.sizeHint().width() + 60)
 
         buttons = QDialogButtonBox(QDialogButtonBox.Save | QDialogButtonBox.Cancel)
         buttons.accepted.connect(self._save)
@@ -355,6 +385,13 @@ class SettingsDialog(QDialog):
     def _model_combo(value: str) -> QComboBox:
         combo = QComboBox()
         combo.setEditable(True)
+        # A combo sizes itself to its longest entry by default; model ids are
+        # long, so the row grew past the dialog and forced a sideways scrollbar
+        # (worse at 125%/150% display scaling). Let it shrink and use a popup
+        # wider than the closed box instead.
+        combo.setSizeAdjustPolicy(QComboBox.AdjustToMinimumContentsLengthWithIcon)
+        combo.setMinimumContentsLength(8)
+        combo.setSizePolicy(QSizePolicy.Ignored, QSizePolicy.Fixed)
         if value:
             combo.addItem(value)
             combo.setCurrentText(value)
@@ -377,6 +414,12 @@ class SettingsDialog(QDialog):
         test_btn.clicked.connect(
             lambda: self._test_connection(self.provider_combo.currentData(), status))
         lay.addWidget(test_btn)
+        # The two buttons keep their natural size; the combo gives way. Without
+        # this the row's minimum was combo + both buttons and nothing could
+        # shrink, so the dialog scrolled sideways instead.
+        for b in (btn, test_btn):
+            b.setSizePolicy(QSizePolicy.Fixed, QSizePolicy.Fixed)
+        row.setSizePolicy(QSizePolicy.Ignored, QSizePolicy.Fixed)
         return row
 
     def _stash_provider_fields(self) -> None: