refactor: replace button+navigate() with React Router NavLink in sidebar

[StarbirdTech]

Fixes #3008

Draft — untested on macOS and Windows. Opening early for visibility and approach feedback.

Replaces the <button onClick={() => navigate(path)}> pattern with React Router <NavLink> for sidebar navigation. The current pattern uses buttons for what are semantically links — screen readers announce "button" instead of "link", there's no right-click "Copy Link" or status bar URL preview, and every component manually reimplements active-state detection and click filtering.

Changes

SidebarItem is now polymorphic — pass a to prop and it renders as <NavLink> with automatic active state and <a> tag semantics. Omit to and it renders as <button> for action items like "Add Location". This is the core of the refactor; everything else follows from it.

Explorer sidebar, LocationsSection, and TagItem now use to prop / <NavLink> directly, eliminating useNavigate(), useLocation(), isActive() helpers, and manual click guards from these components.

shouldNavigate() utility for navigation that can't use <NavLink> — SpaceItem (dnd-kit sortable listeners on the element), TopBarButton expand/back actions, "Open Location" in the inspector. Replaces the incomplete e.button !== 0 guard with full modifier key checks (meta, ctrl, shift, alt).

Not yet tested

I need to check several things on macOS and Windows such as:

• Left-click sidebar NavLinks navigates, active state highlights
• Overview NavLink active only on / (tests end prop)
• Middle-click / Ctrl+click doesn't navigate in-app
• "Add Location" button still works (button mode)
• SpaceItem click + DnD still works
• TopBarButton expand/back in Sync/Jobs popovers still navigates
• DOM inspection: nav items render as <a>, action items as <button>