fix(snackbar): ensure deferred custom snackbar is shown after async gap

[Eimen2018]

Summary

Fixes Stacked-Org/stacked#1163: showCustomSnackBar silently did nothing when called after an await (for example after a delayed API call) instead of from a tap handler or build.

Root cause

SnackbarConfig.instantInit defaults to false. On that path showCustomSnackBar defers getBar.show() to WidgetsBinding.addPostFrameCallback. That callback only runs when the engine next produces a frame. After an async gap with nothing else pending, the binding is idle and no frame is ever scheduled, so the callback never fires and the snackbar never appears. The "two snackbars on a delayed re-tap" from the issue is the same bug: queued callbacks pile up and all fire when an unrelated frame finally runs.

Fix

Call binding.ensureVisualUpdate() right after registering the post-frame callback. It schedules a frame when the binding is idle and is a no-op while a frame is already underway, so the existing "defer until after the current build" behaviour of instantInit: false is unchanged.

Tests

test/snackbar_service_test.dart (new):

• Reproduces the issue: with an idle scheduler, showCustomSnackBar now schedules a frame and the snackbar renders. Verified this test fails on main without the fix.
• Regression guard for instantInit: true.

Notes

• No change to the get dependency (the removal in refactor: remove the get dependency #46 can proceed independently) and no public API change.
• flutter test: 3 passed. dart analyze lib test: no new issues.

🤖 Generated with Claude Code

[Eimen2018]

CI note: the Verify Format failure was pre-existing. dart format --set-exit-if-changed . on main already flags lib/src/dialog/dialog_service.dart and example/lib/ui/views/dialog_view.dart, so every PR fails that step. Added a separate chore commit formatting those two files so this PR can pass. No code change in that commit.

[ferrarafer]

LGTC