refactor(ui): update dialog and welcome tour on QueryaButton

[ZhuchkaTriplesix]

Part of #1338 / #1331 (closes neither)

First feature-directory pass of the button migration, using QueryaButton (#1408, in dev).

What

18 shadcn button calls move to QueryaButton, mapped by the table in docs/ui-kit.md:

Was	Now
PrimaryButton	QueryaButton.primary
OutlineButton	QueryaButton.secondary (ghost when it is Cancel / Close)
GhostButton	QueryaButton.ghost
DestructiveButton	QueryaButton.destructive

• lib/features/updater/update_dialog.dart (12): all footers at lg.
• lib/features/onboarding/welcome_tour_dialog.dart (6): footer and slide actions at lg; the compact Home link (ButtonDensity.compact) is sm; leading: Icon(...) is icon:.
• Keys, labels and callbacks are unchanged; no test looks for a shadcn button type in these two features.

Visible differences (intended, from the standard)

• Cancel in the unsaved-restart dialog loses its outline (ghost).
• Icon sizes follow the control size instead of the hand-set 16 / 18.

Method

The mechanical part was done with a small codemod that only rewrites calls whose arguments it fully understands (key, onPressed, child: Text(...), leading: Icon(...), density, size) and reports the rest; both files converted with nothing skipped, and the diff was read by hand.

Not in this PR

The recovery app (querya_startup_recovery_app.dart) runs outside a QueryaThemeScope and waits for the tolerant QueryaButton of #1418; the toolbars (QueryaActionButton, QueryaActionMenu) are #1337.

Not run locally; CI is the check.