Connections UX: sidebar indent, independent scrolling, selection feedback, long-list handling #254

Problem

Few touch ups needed throughout, lot to improve on...

The Connections page gets worse the more customers it holds. With twenty-odd tenant folders in the sidebar:

• The indent is wrong. Every top-level folder row sits about 90px in from the sidebar edge: a hidden 32px chevron column is reserved for leaves that have no children, on top of list and row padding. The names end up ellipsised while a third of the sidebar is empty space.
• Clicking a customer looks like nothing happened. The sidebar and the entries table share the page scroll. Deep in a long list, the click changes a table that is off screen, so the page appears not to react.
• No independent scrolling. You scroll the whole page to reach a folder, then scroll back up to see its entries. There is no way to keep the list and the content in view at the same time.
• Nothing helps with a long list. No quick filter for folders, no keyboard movement through them, and the selected folder is not brought into view on load.
• Every click re-renders the entire sidebar (renderFolders() runs inside loadEntries() to update one count badge), which flickers and gets slower with the list.

What good looks like

• Sidebar and content scroll on their own: the sidebar stays put and scrolls internally, the entries area scrolls separately, the header stays reachable.
• Selecting a folder scrolls the content to its top and makes the change visible: a highlighted row with an accent bar, a title that visibly changes, a loading state instead of a frozen table.
• Rows flush with the sidebar edge, chevron only where there are children, name and count on one line, description beneath, tighter vertical rhythm so more folders fit without feeling cramped.
• A filter box pinned at the top of the sidebar that narrows the folder list as you type; arrow keys move the selection, Enter opens, Home/End jump; the selected folder is scrolled into view on load.
• Count badges update in place; the sidebar is rebuilt only when the folder set changes.

Branch ux-enhancements, one PR (#255).