Modern UI: sash resize target sits inside panel gap, not on the visible edge

## Feedback

> I want this gap to be smaller. it's also a break of my muscle memory that in order to resize the view, I have to grab _inside_ of gap instead of on the line

**Source**: #workbench-refresh-feedback — 2026-06-26
**Screenshot**: https://slack-files.com/T0HLXG4VA-F0BEAHANC8Y-65a44a2763

---

## Context

With floating panels and the increased outer gutter (`FLOATING_PANEL_MARGIN`), the sash hover/drag area sits visually inside the gap between panels rather than aligned with the visible panel edge. Users expect to grab the visible border line to resize — having to target the interior of a wide gap breaks established muscle memory and makes resizing feel imprecise and hard to discover.

## Acceptance Criteria

- [ ] The sash hit target aligns with the visible panel edge rather than the center of the gap between panels
- [ ] Resizing panels by clicking on their visible border feels natural and matches user expectation
- [ ] The gap width and sash offset are tuned so the grab zone does not feel detached from the panel boundary