Fix tab drop after the last tab in the strip

[parsakhaz]

Description

Dragging a tab to the right of the last tab in a strip (e.g. after Diff/Explorer/Browser in the top bar) was impossible: the drop indicator always meant insert-before-the-hovered-tab, and the strip's trailing empty space was not a drop target, so drops there silently did nothing. This was flagged as a UX gap in the #224 review.

Now the strip follows the VS Code midpoint rule:

• Hovering the left half of a tab inserts before it; the right half inserts after it.
• The empty space after the last tab is a drop target that appends.
• The end-of-strip position shows a right-edge indicator on the last tab; between-tab boundaries keep showing the next tab's left edge, so there is always exactly one indicator.
• Drops commit the midpoint-adjusted index, which composes with the same-group reorder index adjustment in movePanel (dropping a tab on its own right half is a no-op, as expected).

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have run pnpm typecheck and pnpm lint locally

Critical Areas Modified

• None of the above (single component, drag/drop handlers only)