fix(desktop): clear stuck always-on-top after first show on windows

[CannonRS]

Issue for this PR

Closes #46304

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

packages/desktop/src/main/windows.ts shows the main window inside the
ready-to-show handler. On Win32, frameless windows can latch into the
topmost Z-order level on that first show even though the app never called
setAlwaysOnTop(true). The window then behaves as permanently-on-top,
covering every other application the user brings to focus.

This PR adds a single if (process.platform === "win32") win.setAlwaysOnTop(false)
line after win.show() inside the same handler. setAlwaysOnTop(false) is
the documented Win32 fix: it clears the latch without affecting any future
deliberate always-on-top request (the desktop does not currently set
always-on-top itself).

The change is gated on process.platform === "win32" because the latch is a
Win32-only behaviour; on macOS and Linux the call is unnecessary and is
skipped to avoid touching the platform layer.

How did you verify your code works?

• One file, one commit, +5/-0: packages/desktop/src/main/windows.ts.
• bun typecheck in packages/desktop passes (tsgo -b exit 0) after
  bun install --ignore-scripts in the worktree, against upstream/dev
  head 10765ff2a9 plus this commit.
• Manually traced the call site: createMainWindow is the only place in the
  desktop main process that handles the first show; the new line is
  sequenced immediately after win.show() inside the ready-to-show
  callback, so it runs exactly once per window.
• I did not perform a Windows runtime verification in this environment.
  This worktree and the dev container run on a non-Windows host, so a
  physical Windows 10/11 reproduce-and-fix cycle is not possible here. The
  fix is a 1-line API call inside a process.platform === "win32" branch
  and follows the documented Win32 recovery for the topmost-Z-order latch.
  A maintainer with a Windows host should confirm before merge that the
  Z-order latch reproduces on a vanilla dev build and is cleared by the
  new call.

Screenshots / recordings

N/A — no UI code path on this branch, the change is platform-conditional
setAlwaysOnTop(false) inside the desktop main process.

Checklist

• I have tested my changes locally
  • bun typecheck in packages/desktop passes (tsgo -b exit 0) after
    bun install --ignore-scripts in the worktree, against upstream/dev
    head 10765ff2a9 plus this commit. Windows runtime verification is
    blocked in this environment; see "How did you verify your code works?"
• I have not included unrelated changes in this PR