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_dashboard.py

@@ -0,0 +1,92 @@
+"""Check the Dashboard header regroup, on the real widget, offscreen.
+
+Nine controls were on one row. They are now on two, grouped by what they do —
+so this asserts that all nine are still present, still wired, and that the
+header really is two rows now (row 1 = title + Refresh, row 2 = the selectors).
+
+Run:  python tools/check_dashboard.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
+
+HEADER = ["_title", "chart_prev_btn", "_chart_period_lbl", "chart_next_btn",
+          "gran_combo", "metric_combo", "currency_lbl", "currency_combo",
+          "refresh_btn"]
+
+
+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 seed_demo_data import seed
+    seed()
+
+    from cowork_local.i18n import set_language
+    from cowork_local.state import AppContext
+    from cowork_local.ui.dashboard_tab import DashboardTab
+
+    set_language("vi")
+    tab = DashboardTab(AppContext(AppConfig.load()))
+    tab.resize(1100, 800)
+    tab.show()
+    app.processEvents()
+    tab.refresh()
+    app.processEvents()
+
+    fails: list[str] = []
+    missing = [n for n in HEADER if getattr(tab, n, None) is None]
+    print(f"control header con nguyen: {len(HEADER) - len(missing)}/{len(HEADER)}")
+    if missing:
+        fails.append(f"mat control: {missing}")
+
+    # Two rows: everything in the header must sit at one of exactly two y bands.
+    tops = {}
+    for n in HEADER:
+        w = getattr(tab, n)
+        tops.setdefault(round(w.mapTo(tab, w.rect().topLeft()).y() / 10), []).append(n)
+    print(f"so hang cua header       : {len(tops)}")
+    for band, names in sorted(tops.items()):
+        print(f"   y~{band * 10:>4}px : {names}")
+    if len(tops) != 2:
+        fails.append(f"header co {len(tops)} hang, cho 2")
+
+    # Still wired: changing the metric must not throw and must stick.
+    before = tab.metric_combo.currentData()
+    tab.metric_combo.setCurrentIndex(1 - tab.metric_combo.currentIndex())
+    app.processEvents()
+    after = tab.metric_combo.currentData()
+    print(f"doi chi so bieu do       : {before} -> {after}")
+    if after == before:
+        fails.append("combo chi so khong doi duoc")
+    tab.refresh_btn.click()
+    app.processEvents()
+    print("bam Lam moi              : khong loi")
+
+    print()
+    if fails:
+        print("*** LOI ***")
+        for f in fails:
+            print("   " + f)
+        return 1
+    print("KET QUA: header Dashboard chia 2 hang, du 9 control")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())