Replace sidebar MenuList with collapsed MenuBar

[philippjfr]

Summary

The current sidebar uses a pmui.MenuList inside the pmui.Page sidebar drawer, which takes up significant horizontal space even when the user isn't navigating. Since the menu is only needed intermittently, we should switch to a collapsed pmui.MenuBar that saves screen real estate and keeps the focus on the main content area.

However, the sidebar also serves a second purpose: when a dashboard is loaded, components marked with metadata.sidebar are rendered inside a _sidebar_container in the sidebar. The new design must preserve a dedicated area for these dashboard sidebar widgets while still reclaiming the space consumed by the navigation menu.

Current behavior

• pmui.Page(sidebar=...) renders a permanently visible sidebar containing a pmui.MenuList with nested sections (pages grouped by directory, plus "Custom Apps").
• The sidebar is always expanded, consuming ~250px of horizontal space regardless of whether the user is actively navigating.
• When a dashboard is loaded, components with metadata.sidebar=True are placed in self._sidebar_container below the menu (see _rebuild_tile_grid in app.py:542).

Proposed behavior

• Replace the pmui.MenuList navigation with a pmui.MenuBar (collapsed/hamburger-style menu) so navigation doesn't permanently consume horizontal space.
• When a dashboard is active and has sidebar widgets, display them in a slim side panel or drawer that can be toggled independently of the navigation menu.
• When no dashboard sidebar widgets are present, the full viewport width is available for main content.
• Navigation behavior (URL updates, page loading) should remain unchanged.

Implementation notes

• The menu structure is built in get_sidebar() (src/panel_flowdash/app.py:919). The menu_items data structure should be compatible with pmui.MenuBar's API with minimal changes.
• Dashboard action handlers (_on_action_edit, _on_action_rename, etc.) need to be rewired to the new component's action/event system.
• The _refresh_sidebar_dashboards method updates the menu items dynamically, so the new component must support in-place item updates.
• _sidebar_container (a pn.Column) collects sidebar-flagged widget views in _rebuild_tile_grid. The new layout needs to conditionally show a side panel when self._sidebar_views is non-empty.

Acceptance criteria

• Navigation menu is replaced with a collapsed pmui.MenuBar
• All page navigation still works (URL routing, active state)
• Dashboard CRUD actions (create, rename, delete, edit) still function
• Dashboard sidebar widgets still render in a dedicated side area when present
• Main content area uses full width when no sidebar widgets are active and menu is collapsed
• No regressions in existing UI tests

[philippjfr]

Note that MenuBar was just a suggestion.