fix: restore QDrawer swipe-to-open guard inverted in v2.20.0 (fix #18331)

[128na]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

Other information:

Fixes #18331

onOpenPan()'s early-return guard in QDrawer.js was inverted between v2.19.3 and v2.20.0, breaking the edge-swipe-to-open gesture whenever the drawer is closed (exactly the case it's meant to handle).

v2.19.3 (correct):

function onOpenPan(evt) {
  if (showing.value !== false) {
    // some browsers might capture and trigger this
    // even if Drawer has just been opened (but animation is still pending)
    return
  }
  ...

v2.20.0 / v2.20.1 / dev (broken, before this PR):

function onOpenPan(evt) {
  // some browsers might capture and trigger this
  // even if Drawer has just been opened (but animation is still pending)
  if (!showing.value) return
  ...

showing.value !== false (skip only when already showing) was changed to !showing.value (skip when not showing) — the exact logical opposite, so the swipe-to-open gesture became a no-op while the drawer was closed.

This PR reverts the guard back to the v2.19.3 behaviour, with no other functional change.

Reproduction

Live reproduction (broken on current quasar@2, confirmed fixed when pinned to 2.19.3): https://jsfiddle.net/ytma3qLn/

Steps:

1. Open the reproduction link (a q-drawer forced into mobile/swipe mode via breakpoint="9999", starting closed).
2. Drag with the mouse starting at the very left edge (x < 15px) towards the right.
3. Without this fix: nothing happens. With this fix (or quasar@2.19.3): the drawer opens.

[rstoenescu]

Ahhhh.. saw the PR after fixing it myself. Sorry :(
But thank you for the bug report and the PR!