fix: add position:relative to .interactive-session for dnd overlay

[octo-patch]

Fixes #925

Problem

Dragging files from the Explorer to the Chat panel does not show the blue drop overlay. The .chat-dnd-overlay element uses position: absolute to cover the chat session container, but its parent .interactive-session was missing position: relative. Without a positioning context on the parent, the absolutely-positioned overlay is anchored to a wrong ancestor element (e.g., the viewport), causing it to render off-screen with no visible feedback during drag operations.

Solution

Add position: relative to .interactive-session in chat.css. This provides the correct positioning context so .chat-dnd-overlay is sized and positioned relative to the chat panel rather than a distant ancestor.

/* Before */
.interactive-session {
    max-width: 850px;
    margin: auto;
}

/* After */
.interactive-session {
    max-width: 850px;
    margin: auto;
    position: relative;
}

Testing

• Drag a file from the Explorer panel into the Chat panel — the blue overlay now appears correctly
• Existing drag-and-drop logic in chatDragAndDrop.ts is unchanged; this is a one-line CSS-only fix
• No regressions to layout: position: relative without explicit top/left/width/height has no visual side effects on the container itself