Preserve thread text when offline saves are partial

[CHOS1N11111]

Summary

• persist the currently displayed main post and loaded replies before additional network work
• keep successfully fetched replies and nested replies when individual pages fail
• skip failed media items while retaining the text snapshot and marking the save as partial
• preserve an existing saved version when an update cannot improve it
• store partial-capture status in a backward-compatible snapshot format and surface it in offline reading

Root cause

The previous save flow fetched every reply and nested reply, downloaded every requested media item, and only then wrote the snapshot. Any request, consistency check, media download, or size failure discarded the entire operation even when the main-post text was already visible.

Testing

• added coverage for visible-content baselines, missing main-post ids, partial status round trips, v2 compatibility, result messages, and best-effort request/media policies
• GitHub Actions passed unit tests, release lint, debug/release APK builds, release AAB build, and release output verification
• local Windows Gradle execution remains blocked before project configuration by a Java loopback connection error