[Lng999]

Custom mods now have selectable slots above the hovered skin carousel, with the selected mod's preview shown on its targeted skin art. Switching from mod A to B, deselecting, or leaving champion select restores the appropriate original art.

Supersedes #342, updated on current main with its review fixes preserved. Preview handling supports video elements with source children, layered backgrounds, and mods targeting multiple skins. The HistoricMode popup is suppressed only while the selected slot is visible; failed champion-alias lookups retry, and inactive phases stop preview scans. Mods responses carry the authoritative backend selection so unlocking/relocking or loading mid-session restores the selected slot and preview without an extra state broadcast.

This replacement also fixes stale lock state after reconnecting and refreshes slot names and thumbnails when metadata changes without changing mod identifiers. Regression tests exercise the backend snapshot and real plugin logic through simulated bridge events and DOM updates. No live League client was tested.

Validation:

• Full isolated Python suite: 361 passed.
• JavaScript regressions: 15 passed (10 Custom Slots, 5 Party Mode).
• Node syntax checks, scoped CI Ruff checks, and git diff --check passed.