feat: swipe-from-edge to open the sidebar on mobile #292

On mobile the sidebar opens only via the toggle button in the top bar. The sidebar component has no touch handling at all; the only pointer logic in it is the desktop resize handle, which is explicitly disabled below the md breakpoint.

Open WebUI's mobile UI has a slide-to-open gesture: swiping in from anywhere along the left edge opens the sidebar, and it can also be swiped closed. That is the expected mobile pattern (same as the platform drawer gestures) and makes the sidebar reachable from anywhere on the screen without reaching for the top-left corner.

Suggested change: a left-edge swipe gesture on mobile (below the md breakpoint) that opens the sidebar, tracking the finger so the sidebar follows the drag, plus swipe-to-close. Details that make or break it:

• start the gesture only from a narrow left-edge zone so horizontal scrolling inside content is not hijacked
• ignore swipes that start on interactive elements
• make the sidebar follow the finger during the drag and settle open/closed on release, rather than a threshold-only trigger

The sidebar is already a fixed overlay on mobile with a backdrop, so this is gesture handling on top of the existing structure, no layout rework.