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/task_editor_dialog.py

@@ -86,12 +86,18 @@ class TaskEditorDialog(QDialog):
         outer = QVBoxLayout(self)
         scroll = QScrollArea()
         scroll.setWidgetResizable(True)
+        scroll.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)   # down only
         content = QWidget()
         scroll.setWidget(content)
         outer.addWidget(scroll, 1)
         root = QVBoxLayout(content)
 
         # ---- basics ----------------------------------------------------
+        # Zero-height anchor: this block is a bare form, so the index needs
+        # something to scroll to.
+        self._anchor_basic = QWidget()
+        self._anchor_basic.setFixedHeight(0)
+        root.addWidget(self._anchor_basic)
         form = QFormLayout()
         self.title_edit = QLineEdit(self.task.get("title", ""))
         # Description is the source of truth. Its ✨ button GENERATES the Prompt
@@ -427,6 +433,26 @@ class TaskEditorDialog(QDialog):
         eform.addRow("", self.approval_chk)
         root.addWidget(eg)
 
+        # Five groups in one long scroll — same problem, same fix as Settings.
+        from .widgets import section_index
+        self.section_list = section_index(scroll, [
+            (tr("schedtask.g_basic"), self._anchor_basic),
+            (tr("schedtask.g_schedule"), sg),
+            (tr("schedtask.g_input"), ig),
+            (tr("schedtask.g_dependency"), dg),
+            (tr("schedtask.g_execution"), eg),
+        ])
+        outer.removeWidget(scroll)
+        body = QHBoxLayout()
+        body.setSpacing(10)
+        body.addWidget(self.section_list)
+        body.addWidget(scroll, 1)
+        outer.insertLayout(0, body, 1)
+        # Same floor as Settings: the dialog may not be narrower than its own
+        # content at the font in use, so nothing is ever cut off sideways.
+        self.setMinimumWidth(self.section_list.width()
+                             + content.sizeHint().width() + 60)
+
         buttons = QDialogButtonBox(QDialogButtonBox.Save | QDialogButtonBox.Cancel)
         buttons.button(QDialogButtonBox.Save).setIcon(icon("save"))
         buttons.button(QDialogButtonBox.Cancel).setIcon(icon("close"))