Add config option to use local video paths in gamelist.xml export

[JamieeBond]

Relating to #3069

Description
Add config value to use local videos in the gamelist.xml export, instead of YouTube, if set to true.

In the config, i created it as a new config array, to add some future proofing, rather than a standalone key, since gamelist is relevantly new, so config settings for the export or import can have a place to live:

 gamelist:
   export:
     local_video: true

Uses the same logic as here, to decide which video to use:

romm/frontend/src/composables/useGameAnimation.ts

Lines 195 to 202 in 527a45d

	const localVideoPath = computed(() => {
	if (!romsStore.isSimpleRom(rom)) return null;
	// Only play video if boxart style is miximage
	if (boxartStyle.value !== "miximage_path") return null;
	const ssVideo = rom.ss_metadata?.video_path;
	const gamelistVideo = rom.gamelist_metadata?.video_path;
	return ssVideo || gamelistVideo || null;
	});

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

[greptile-apps]

Greptile Summary

This PR modifies backend/utils/gamelist_exporter.py to prefer locally stored video files (from ss_metadata.video_path or gamelist_metadata.video_path) over YouTube embed links when building the <video> element of a gamelist.xml export, falling back to YouTube only when no local path exists.

Key issue:

• The PR title and description promise a config-gated option (gamelist.export.local_video: true) that lets users opt in to local video paths. The config flag was never implemented — no GAMELIST_EXPORT_LOCAL_VIDEO attribute was added to Config, _parse_config(), or _validate_config() in config_manager.py. The new behaviour is therefore unconditional, silently overriding YouTube video links with local paths for all existing installations, regardless of user preference.

Positive aspects:

• The local-path resolution logic mirrors the existing frontend logic in useGameAnimation.ts (lines 195-201), keeping behaviour consistent between the UI and the export.
• The fallback chain (ss_metadata → gamelist_metadata → YouTube) is sensible.

Confidence Score: 2/5

• Not safe to merge — the advertised config gate was not implemented, making this an unconditional breaking behaviour change.
• The core intent of the PR (an opt-in config flag) is entirely absent from the implementation. All users will have their gamelist.xml video links silently switched from YouTube to local paths without any opt-in, which is the opposite of the described design. The config manager, its parsing, and validation are all untouched. Both backend/utils/gamelist_exporter.py and backend/config/config_manager.py need attention — the exporter needs the config flag guard, and the config manager needs the new GAMELIST_EXPORT_LOCAL_VIDEO attribute, parser entry, and validator.
• backend/utils/gamelist_exporter.py (needs config guard), backend/config/config_manager.py (needs GAMELIST_EXPORT_LOCAL_VIDEO attribute, parser, and validator)

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["_create_game_element(rom)"] --> B{ss_metadata\nvideo_path?}
    B -- yes --> C["video = FRONTEND_RESOURCES_PATH/ss_metadata.video_path"]
    B -- no --> D{gamelist_metadata\nvideo_path?}
    D -- yes --> E["video = FRONTEND_RESOURCES_PATH/gamelist_metadata.video_path"]
    D -- no --> F{youtube_video_id?}
    F -- yes --> G["video = YOUTUBE_BASE_URL/embed/youtube_video_id"]
    F -- no --> H["No video element added"]

    style B fill:#f9a,stroke:#c33
    style D fill:#f9a,stroke:#c33
    note1["⚠️ Config flag\n(gamelist.export.local_video)\ndescribed in PR\nbut NOT implemented"]
    note1 -.-> B

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_{Last reviewed commit: fcf015e}