fix timeline dup keys

[Tlaster]

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[Copilot]

Pull request overview

This PR addresses duplicate Compose lazy item keys in mixed timelines by making UiTimelineV2.itemKey account-scoped and by deduplicating mixed-timeline results before they are returned/saved.

Changes:

• Add accountType to UiTimelineV2.itemKey generation to avoid key collisions across accounts.
• Deduplicate MixedRemoteMediator merged results with distinctBy(itemKey) after sorting.
• Add regression tests covering deduplication across sub-timelines and preserving same-status items from different accounts.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
shared/src/commonTest/kotlin/dev/dimension/flare/data/datasource/microblog/MixedRemoteMediatorTest.kt	Adds tests to validate mixed refresh deduplication behavior and cross-account uniqueness.
shared/src/commonMain/kotlin/dev/dimension/flare/ui/model/UiTimelineV2.kt	Updates itemKey to include accountType, preventing collisions between different accounts.
shared/src/commonMain/kotlin/dev/dimension/flare/data/datasource/microblog/MixedRemoteMediator.kt	Deduplicates merged timeline items by itemKey to prevent duplicate lazy keys in mixed timelines.

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