chore(desktop): spaces dragging fix + dev flag

[tatoalo]

Problem

window dragging + dev flag for spaces layout experimentation

before:

nah.mp4

after:

yay.mp4

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 6b590de.}

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
packages/ui/src/features/canvas/hooks/useChannelsLayout.ts:12-15
**Development fallback contradicts comment**

Both feature-flag checks now use `import.meta.env.DEV` as their fallback, while the adjacent documentation still says there is no development default and that development matches production. This contradiction makes the gate's intended behavior harder to understand and maintain.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "chore(desktop): spaces dragging fix + de..." | Re-trigger Greptile}

[github-actions]

Tiny, contained UI/dev-flag tweak (drag-region class fix + dev-flag fallback) with no risky territory touched; the only outstanding comment is a cosmetic doc-consistency nit, not a functional concern.

• greptile-apps[bot] reviewed the current head.
• Greptile inline comment notes the adjacent doc comment now contradicts the DEV fallback behavior — cosmetic, non-blocking, but worth a follow-up tweak.

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	9L, 2F substantive — within ceiling
tier	✓	T1-agent / T1a-trivial (9L, 2F, single-area, chore)
stamphog 2.0.0b3		.stamphog/policy.yml @ 7d1d9d7 · reviewed head a4ae51b