refactor: simplify ui stability changes

[ovitrif]

This PR simplifies the immutable collections refactor (#839) by removing redundant conversions, fixing view-layer violations, and cleaning up preview data patterns.

Description

1. Removes redundant .toImmutableList()/.toImmutableSet() calls on already-immutable data (BlocktankState fields, selectedTags)
2. Moves view-layer conversions to source layer (ActivityDetailViewModel, SettingsViewModel, LightningConnectionsViewModel)
3. Converts previewActivityItems to ImmutableList at source, eliminating 8 call-site conversions
4. Replaces listOf().toImmutableList() with persistentListOf() in previews
5. Reverts ToastQueueManager internal queue to plain List (not exposed to Compose)
6. Extracts repeated ActivityTab.entries.toImmutableList() into file-scope vals

Preview

N/A — no visual changes, refactoring only.

QA Notes

1. Regression Checks

1. Open the app and navigate through wallet screens (Home, Savings, Spending)
2. Verify activity lists display correctly
3. Open Lightning Connections settings and verify channels display in correct order
4. Verify tag selector and activity filtering work as expected

Note for reviewer: Relying on e2e suite should be enough, PR changes were tested manually, and it's only updating coding cosmetics of previous PR.