revert(v2): hydrate gallery cards per-card via /roms/{id}/simple

[gantoine]

Description
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The v2 gallery previously hydrated each visible card with its own GET /roms/{id}/simple request. Because that endpoint was slow at the time, a prior change (#3765) switched hydration to batched 72-item window fetches through the paginated /roms list endpoint.

Now that /roms/{id}/simple is fast (lightweight eager-load), this reverts the gallery back to per-card hydration:

• galleryRoms store — reintroduces fetchRomAt(position) (per-card GET /roms/{id}/simple) and cancelFetchAt(position). Removes the now-unreachable window-fetch machinery: fetchWindowAt, syncVisibleWindows, cancelWindow, the window auto-retry (retryTimers/retryCounts/backoff), the batched-apply helpers (applyItemsBatched/nextFrame), alignToWindow, the loadedWindows/pendingWindows/failedWindows state, and the unused hasInitial getter.
• GalleryShell — grid-mode viewport sync fires fetchRomAt for entered positions and cancelFetchAt for departed ones, restoring the visiblePositions tracking set.
• GameListRow — row mount fires fetchRomAt, unmount aborts via cancelFetchAt.

Kept from the intervening work, as intended:

• the withCharIndex / withFilterValues skip params on the roms API (services/api/rom.ts untouched),
• the abortInFlight store action, still wired into the shell's unmount.

The lightweight fetchInitialMetadata bootstrap (seeds total / charIndex / romIdIndex / filter_values) is unchanged.

AI assistance disclosure: This PR was written with AI assistance (PostHog Code / Claude). The change was reviewed by the author.

Checklist
^{Please check all that apply.}

• 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

Screenshots (if applicable)

No UI/visual change; behavior-only revert of the hydration path.

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Created with PostHog Code

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[greptile-apps]

Greptile Summary

This PR restores per-card hydration for the v2 gallery. The main changes are:

• Fetches visible grid cards through /roms/{id}/simple.
• Starts and cancels hydration from list-row lifecycle hooks.
• Removes window batching, retry, and window-state bookkeeping.

Confidence Score: 4/5

The per-card hydration lifecycle can leave visible grid and list cards stuck as skeletons.

• Cancelled requests can delete the controller owned by a newer request for the same position.
• Transient request failures have no retry while the viewport remains still.
• Mounted list rows do not refetch after gallery state is invalidated at an unchanged position.

frontend/src/v2/stores/galleryRoms.ts and frontend/src/v2/components/Gallery/GameListRow.vue

Important Files Changed

Filename	Overview
frontend/src/v2/stores/galleryRoms.ts	Adds per-position hydration and cancellation, but controller cleanup and failed-request handling can leave visible cards unhydrated.
frontend/src/v2/components/Gallery/GalleryShell.vue	Replaces window synchronization with debounced per-position fetching and cancellation for the grid.
frontend/src/v2/components/Gallery/GameListRow.vue	Moves list hydration to mount and unmount hooks, which misses refreshes when a position-keyed row remains mounted.

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