Fix side terminal tab resizing

[Finesssee]

Summary

• Fix vertical terminal tab sidebar resizing when tabs are aligned on the right side
• Move the resize handle to the inner edge for right-aligned side tabs
• Add focused coverage for left and right sidebar resize math

Fixes #689

Issue

#689 reports that terminal alignment dragging does not work properly in sideview. The broken case is the vertical terminal tab sidebar after moving tabs to the right side: dragging the divider behaves backwards/incorrectly because the sidebar still acts like it is on the left side.

Reproduction

Yes, reproduced against latest master before applying the fix. This branch was created directly from upstream master at 6814bdd6 (Stabilize Windows language tool installs). In that state, the vertical terminal tab sidebar still used the right-edge resize handle and left-to-right width delta even when tabs were placed on the right side.

Fix

When terminal tabs are on the right, the resize handle now sits on the sidebar's left edge and the drag delta is inverted. Left-side tabs keep the existing right-edge behavior.

Testing

• bunx vp test run src/features/terminal/tests/terminal-tab-sidebar-resize.test.ts
• bun typecheck
• git diff --check