[Settings][Image Resizer] Edit/add presets in a ContentDialog instead of a Flyout

[niels9001]

Summary of the Pull Request

Replaces the inline preset-edit Flyout on the Image Resizer settings page with a ContentDialog, matching the add/edit pattern already used on the Color Picker page. This aligns the experience with the Windows 11 / Fluent paradigm and fixes preset settings being saved on every intermediate change.

Editing now happens on a working copy of the preset (ImageSize.Clone()), which is committed only when the user presses Save/Update. As a side effect, the intermediate width/height spinner changes no longer persist settings.json / sizes.json on every value change — resolving #36938.

The per-row delete action moves from an inline trash button into a "..." (More options) MenuFlyout, again matching the Color Picker page.

Historically this editing was a Flyout rather than a ContentDialog due to known ContentDialog / XamlRoot issues back when Settings was a UWP app. Now that Settings is on WinUI 3 / Windows App SDK, ContentDialog works reliably (as Color Picker's ColorFormatDialog demonstrates), so the original constraint no longer applies.

ImageResizer.mp4

PR Checklist

• Closes: [Settings][Image Resizer] Edit/add presets in a ContentDialog instead of a Flyout #49157
• Closes: [Settings] ImageResizer settings files may be saved too often #36938
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized (added to Resources.resw; reused existing keys for the delete menu)
• Dev docs: Added/updated
• New binaries: None added

Detailed Description of the Pull Request / Additional comments

• Dialog: Clicking a preset card (or Add new size) opens an EditSizeDialog ContentDialog. Fields are bound with compiled {x:Bind} against a working-copy EditingSize. The dimensions field header is dynamic — "Width" when height is used, "Size" for aspect-ratio-preserving percentage scaling — so the label isn't misleading. The dialog is widened for a less cramped layout.
• Save semantics:
  • ImageSize.Clone() — builds the editable working copy.
  • ImageResizerViewModel.CreateNewImageSizeModel() — builds a default-valued model for the add dialog without adding it to Sizes.
  • ImageResizerViewModel.AddImageSize(ImageSize) — commits a new preset with the next unique ID.
  • ImageResizerViewModel.UpdateImageSize(original, updated) — applies edited values back onto the original, temporarily detaching the per-item PropertyChanged save handler so it persists once instead of on every property. This is what fixes the "saved too often" behavior in [Settings] ImageResizer settings files may be saved too often #36938.
• Delete: per-row Button → MenuFlyout with a Delete MenuFlyoutItem; the ImageSize is passed via CommandParameter="{x:Bind}" (robust inside a flyout popup) and the Yes/No confirmation dialog is preserved.

Validation Steps Performed

• Built PowerToys.Settings (x64/Debug) — clean (exit 0).
• Ran the runner from this build and manually validated in Settings → Image Resizer:
  • Add new size opens the dialog pre-filled; Save adds the preset; Cancel discards.
  • Editing a preset in the dialog and pressing Cancel leaves the original untouched (working-copy clone).
  • Selecting Percent shows the Size header (not a misleading "Width").
  • Spinning width/height inside the dialog no longer writes settings files on each change; a single save occurs on Update ([Settings] ImageResizer settings files may be saved too often #36938).
  • The "..." menu shows Delete, with the confirmation dialog intact.