fix(v2): clear gallery selection after deleting ROMs

[gantoine]

Description
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Fixes #3822.

In the v2 UI, deleting selected games left them "selected": multiselect mode stayed active, the SelectionBar stayed open pointing at rows that no longer exist, and re-running a bulk action (delete again, download, ...) returned a "not found" error.

Root cause: DeleteRomDialog.vue's deleteRoms() cleared only the legacy v1 selection store (romsStore.resetSelection()). v2 multiselect and the SelectionBar read from a separate store, v2GallerySelection (frontend/src/v2/stores/gallerySelection.ts), which was never touched, so the deleted ROMs stayed in the selection set (enabled = size > 0 keeps the bar open).

Changes:

• Frontend: after a delete, prune the deleted ROMs from the v2 gallery selection (gallerySelection.removeIds).
• Partial-failure correctness: the bulk delete endpoint deletes per-ROM and can partially fail. The endpoints (/roms/delete and /firmware/delete, both via BulkOperationResponse) now return failed_ids, and the client only prunes the ROMs that were actually removed, so a failed subset stays visible and selected for retry. Only the succeeded IDs are removed from the gallery / recent / continue-playing lists, and the single-ROM route only navigates away when its ROM was deleted.
• Concurrency safety: the handler snapshots the dialog state at entry and acts on that snapshot, so a showDeleteRomDialog event arriving mid-request can't make the completed request prune the wrong IDs.
• Tests: added test_delete_roms_reports_failed_ids asserting the failed id is reported while the valid one is deleted.

AI assistance disclosure: This change was written with AI assistance (Claude Code), reviewed by the author.

Checklist
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• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

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🤖 Generated with Claude Code

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[greptile-apps]

Greptile Summary

This PR synchronizes ROM deletion with the v2 gallery selection. The main changes are:

• Adds the v2 gallery-selection store to the delete dialog.
• Removes requested ROM IDs from v2 selection after deletion.

Confidence Score: 4/5

The selection reconciliation needs to preserve failed deletions and use the original request IDs.

• Partial bulk failures remove ROMs from selection even though they remain in the gallery.
• Reading mutable dialog state after the request can clear IDs from a later dialog invocation.

frontend/src/v2/components/Dialogs/DeleteRomDialog.vue

Important Files Changed

Filename	Overview
frontend/src/v2/components/Dialogs/DeleteRomDialog.vue	Reconciles v2 gallery selection after deletion, but can clear failed or unrelated ROM IDs.

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