feat: add loading widget support for image and avatar components

[usmanvrtx]

Description

This PR adds a loadingWidget property to the Image and Avatar components so developers can display a custom widget while a network image is loading. Previously only a colored placeholder box could be shown during the loading state. It also introduces a networkCacheManager option that lets developers supply a custom cache manager for network image requests.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

What Has Changed

• Added loadingWidget and networkCacheManager fields, getters, and setters to the EnsembleImage and Avatar controllers
• When loadingWidget is set, it is built through the scope manager and shown as the placeholder while a network image loads; otherwise the existing colored placeholder is used
• Refactored image placeholder logic into reusable _buildLoadingPlaceholder / _buildLoadingWidget helpers in widget/image.dart
• Extended the BackgroundImage model with loadingWidget and cacheManager support
• Threaded loadingWidget through the shared framework Image widget's placeholder builder
• Added networkCacheManager support so a custom BaseCacheManager can be passed for network image loading

How to Test

1. Add loadingWidget to an Image or Avatar definition and load a slow network image — the custom widget should render while loading
2. Verify the existing placeholderColor placeholder still renders when loadingWidget is not set
3. Optionally pass a networkCacheManager and confirm network images use it
4. Run the existing image tests: flutter test modules/ensemble/test/widget/image_widget_test.dart

Screenshots / Videos

N/A

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors

[TheNoumanDev]

Following is Claude Review, please go though each point and see if it make sense, fix it, and if doesn't reply on it, Thanks

🔧 What we're fixing (the two that actually bite)

- #1 — Permanent spinner on empty source (widget/image.dart:132)
  - Empty/absent source now shows loadingWidget forever, because that branch is the terminal "no image" state, not a real loading state — no fetch is pending to replace it.
  - Fix: keep the empty-source branch as a neutral blank placeholder; only show loadingWidget on the real network-loading path.
- #2 — Loading widget rebuilt every frame (widget/image.dart:293)
  - The placeholder callback calls buildWidgetFromDefinition(...) on every rebuild → animated loaders reset, and any id in the loader gets re-registered in scope repeatedly.
  - Avatar already does it right (builds once, reuses). Fix: build once and reuse here too.

📋 The remaining points (lower priority, worth cleaning up)

- #3 — Redirect blocking silently dropped (image.dart:288, avatar.dart:316) — if a caller supplies networkCacheManager and allowRedirect:false, the redirect guard is bypassed. Only reachable programmatically today, but it voids a security setting.
- #4 — networkCacheManager YAML setter is a no-op (image.dart:73, avatar.dart:82) — BaseCacheManager can't be built from YAML, so any YAML value becomes null silently. Document as native-only or remove.
- #5 — Avatar lost {super.key} (avatar.dart:27) — now the only EnsembleWidget subclass that can't take a key. No caller passes one today, so latent — looks like accidental collateral; revert.
- #6 — BackgroundImage params are dead plumbing (model.dart:62) — loadingWidget/cacheManager were added to the model but getBackgroundImage never passes them, so they're always null. Wire up or drop.