feat: add sidebar markup, toggle button, and view-body wrapper to expanded view

- Add #sidebar-toggle-btn to expanded-header (after #back-btn, before #expanded-session-name)
  with aria-label='Toggle session list' and hamburger icon ☰
- Wrap #terminal-container in div.view-body
- Add #session-sidebar before #terminal-container inside .view-body, containing:
  - .sidebar-header with .sidebar-title 'Sessions' and #sidebar-collapse-btn (chevron ‹)
  - #sidebar-list div
- Keep #reconnect-overlay outside .view-body as sibling inside #view-expanded

Tests added:
- test_html_sidebar_toggle_button: verifies placement, aria-label, and order in header
- test_html_view_body_wrapper: verifies .view-body contains #session-sidebar and #terminal-container
- test_html_reconnect_overlay_outside_view_body: verifies #reconnect-overlay is outside .view-body
- test_html_session_sidebar_structure: verifies full sidebar DOM structure

Co-authored-by: Amplifier <amplifier@amplified.dev>

coordinator/tests/test_frontend_html.py

@@ -133,6 +133,90 @@ def test_html_style_css() -> None:
     )
 
 
+def test_html_sidebar_toggle_button() -> None:
+    """#sidebar-toggle-btn must exist in expanded-header with correct aria-label and hamburger icon."""
+    soup = _SOUP
+    btn = soup.find(id="sidebar-toggle-btn")
+    assert btn is not None, "Missing #sidebar-toggle-btn"
+    assert btn.get("aria-label") == "Toggle session list", (
+        f"#sidebar-toggle-btn aria-label must be 'Toggle session list', got: {btn.get('aria-label')!r}"
+    )
+    # Must be inside expanded-header
+    header = soup.find("header", class_="expanded-header")
+    assert header is not None, "Missing header.expanded-header"
+    assert header.find(id="sidebar-toggle-btn") is not None, (
+        "#sidebar-toggle-btn must be inside header.expanded-header"
+    )
+    # Must be after #back-btn and before #expanded-session-name
+    header_children_ids = [el.get("id") for el in header.children if hasattr(el, "get")]
+    header_children_ids = [i for i in header_children_ids if i]
+    assert "back-btn" in header_children_ids, "#back-btn must be in expanded-header"
+    assert "sidebar-toggle-btn" in header_children_ids, "#sidebar-toggle-btn must be in expanded-header"
+    assert "expanded-session-name" in header_children_ids, "#expanded-session-name must be in expanded-header"
+    back_idx = header_children_ids.index("back-btn")
+    toggle_idx = header_children_ids.index("sidebar-toggle-btn")
+    name_idx = header_children_ids.index("expanded-session-name")
+    assert back_idx < toggle_idx < name_idx, (
+        f"Order must be back-btn < sidebar-toggle-btn < expanded-session-name, got indices {back_idx}, {toggle_idx}, {name_idx}"
+    )
+
+
+def test_html_view_body_wrapper() -> None:
+    """.view-body div must exist inside #view-expanded wrapping #session-sidebar and #terminal-container."""
+    soup = _SOUP
+    view_expanded = soup.find(id="view-expanded")
+    assert view_expanded is not None, "Missing #view-expanded"
+    view_body = view_expanded.find("div", class_="view-body")
+    assert view_body is not None, "Missing div.view-body inside #view-expanded"
+    # #terminal-container must be inside .view-body
+    assert view_body.find(id="terminal-container") is not None, (
+        "#terminal-container must be inside div.view-body"
+    )
+    # #session-sidebar must be inside .view-body
+    assert view_body.find(id="session-sidebar") is not None, (
+        "#session-sidebar must be inside div.view-body"
+    )
+
+
+def test_html_reconnect_overlay_outside_view_body() -> None:
+    """#reconnect-overlay must be a direct child of #view-expanded, NOT inside .view-body."""
+    soup = _SOUP
+    view_expanded = soup.find(id="view-expanded")
+    assert view_expanded is not None, "Missing #view-expanded"
+    view_body = view_expanded.find("div", class_="view-body")
+    assert view_body is not None, "Missing div.view-body"
+    # reconnect-overlay must NOT be inside view-body
+    assert view_body.find(id="reconnect-overlay") is None, (
+        "#reconnect-overlay must NOT be inside div.view-body"
+    )
+    # reconnect-overlay must be inside view-expanded (as sibling of view-body)
+    assert view_expanded.find(id="reconnect-overlay") is not None, (
+        "#reconnect-overlay must be inside #view-expanded"
+    )
+
+
+def test_html_session_sidebar_structure() -> None:
+    """#session-sidebar must contain .sidebar-header (with .sidebar-title and #sidebar-collapse-btn) and #sidebar-list."""
+    soup = _SOUP
+    sidebar = soup.find(id="session-sidebar")
+    assert sidebar is not None, "Missing #session-sidebar"
+    # .sidebar-header
+    sidebar_header = sidebar.find(class_="sidebar-header")
+    assert sidebar_header is not None, "Missing .sidebar-header inside #session-sidebar"
+    # .sidebar-title with text 'Sessions'
+    sidebar_title = sidebar_header.find(class_="sidebar-title")
+    assert sidebar_title is not None, "Missing .sidebar-title inside .sidebar-header"
+    assert "Sessions" in sidebar_title.get_text(), (
+        f".sidebar-title text must contain 'Sessions', got: {sidebar_title.get_text()!r}"
+    )
+    # #sidebar-collapse-btn
+    collapse_btn = sidebar_header.find(id="sidebar-collapse-btn")
+    assert collapse_btn is not None, "Missing #sidebar-collapse-btn inside .sidebar-header"
+    # #sidebar-list
+    sidebar_list = sidebar.find(id="sidebar-list")
+    assert sidebar_list is not None, "Missing #sidebar-list inside #session-sidebar"
+
+
 def test_html_element_classes() -> None:
     """Critical and important elements must carry their CSS styling classes."""
     soup = _SOUP