refactor(CardTemplateEditor): add ViewModel + State infrastructure

[xenonnn4w]

Purpose / Description/ Approach

This PR adds the foundational infrastructure without changing existing behavior:

• CardTemplateEditorState: Immutable data class holding UI state (loading, current template ordinal, editor view, errors, messages)
• CardTemplateEditorViewModel: Manages state via StateFlow with thread-safe updates
• CardTemplateEditorViewModelTest: Unit tests covering all state operations

This is Part 1 of a multiPR migration:

PR 1 (this): Infrastructure (ViewModel + State) [Done]
PR 2: Wire ViewModel to UI
PR 3: Load notetype + Template manipulation in-memory
PR 4: Atomic save with updateNotetype()
PR 5: Remove legacy code

Links without closing (Part of)

• Migrate Card Template Editor to Rust backend for atomic notetype updates #19956

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[david-allison]

I don't really understand what the abstraction will turn out to be.

Mostly seems fine, even if it's just starting to move the screen to a ViewModel

[david-allison]

I don't want to block forward progress on nitpicks, especially as this change does not affect production functionality.

This should be the last 'change request' before an approve.

Thanks!!!

[david-allison]

Looks like a great start, thanks!