Support for Caching on Web using IndexedDB

[ben-milanko]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Feature

:arrow_heading_down: What is the current behavior?

Flutter web has no persistent caching. Cache is stored only in memory and lost on page refresh.

:new: What is the new behavior (if this is a feature change)?

Cache metadata and files stored in IndexedDB. Data persists across page refreshes and browser sessions.

:boom: Does this PR introduce a breaking change?

No - Non-breaking change. Works automatically without code changes. SDK 3.6.0+ now required.

:bug: Recommendations for testing

1. Switch test app to use the repo in this PR
2. Download files in app
3. Refresh app and observe new caching behaviour
4. Check IndexedDB in Chrome DevTools

:memo: Links to relevant issues/docs

#480 #339 #361 #122

:thinking: Checklist before submitting

• [ ] All projects build - Appears to be issues getting the example builds to work, but I have tested it in my apps that use flutter_cache_manager and cached_network_image
• [ ] Follows style guide lines (code style guide)
• [ ] Relevant documentation was updated
• [ ] Rebased onto current develop