Surface never-scanned platform folders in the scan picker

[gantoine]

Description
^{Explain the changes or enhancements you are proposing with this pull request.}

Fixes #3832.

A platform folder created on disk (and optionally bound in Library Management) has no database row until its first scan imports a ROM, so it was invisible to the Scan platform picker and a first scan could not be triggered for it. The picker also filtered to rom_count > 0, hiding even zero-ROM database platforms (e.g. ones created via romm-cli). The backend scanner already works in folder-slug space, so the gap was purely that a never-scanned folder could not be seen or targeted.

Backend

• utils/platforms.py: extracted the shared "build a PlatformSchema from local metadata" logic into _build_unmatched_platform(), and added get_filesystem_platforms() returning folders on disk without a database row yet (slug resolved via PLATFORMS_BINDING/PLATFORMS_VERSIONS, id: -1, rom_count: 0, no network calls).
• endpoints/platform.py: new GET /platforms/filesystem (scope PLATFORMS_READ), declared before /{id}.
• endpoints/sockets/scan.py: scan_platforms() and the scan socket handler accept platform_fs_slugs. Selected database platforms (by id or slug) are always honored; a bare slug is accepted when it maps to a folder on disk; an empty selection still scans everything.

Frontend (v2, the default UI)

• services/api/platform.ts + stores/platforms.ts: getFilesystemPlatforms(), a filesystemPlatforms state slice, fetchFilesystemPlatforms(), and a scannablePlatforms getter (all database platforms including zero-ROM plus unscanned folders).
• v2/components/shared/PlatformSelect.vue: supports item-key="fs_slug" and an opt-in mark-unscanned + unscanned-label marker for id < 0 folders.
• v2/views/Scan.vue: the picker is keyed by fs_slug, fetches filesystem folders on mount, tags never-scanned folders, and emits platform_fs_slugs.
• Added the scan.folder-not-scanned key, translated across all 18 locales.

v1 (src/views/Scan.vue) is frozen and left unchanged; it keeps working against the backwards-compatible backend (it still sends platform ids).

AI assistance disclosure

This PR was written with AI assistance (Claude Code, Opus 4.8). The implementation, tests, and translations were AI-generated and 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)

N/A — verified via typecheck, backend + frontend test suites, i18n parity/sort checks, and Trunk. A full in-browser pass (both themes / input modalities) was not run and is recommended before merge.

🤖 Generated with Claude Code

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[greptile-apps]

Greptile Summary

This PR makes never-scanned platform folders available in the v2 scan picker. The main changes are:

• Adds a protected endpoint for filesystem-only platform folders.
• Accepts filesystem slugs when starting a scan.
• Includes zero-ROM and never-scanned platforms in the v2 picker.
• Adds an unscanned-folder marker across all supported locales.

Confidence Score: 4/5

Explicit invalid selections can start a full scan, and completed scans leave stale duplicate picker entries.

• Valid ID and filesystem-slug selections remain compatible.
• A stale or renamed selected folder can expand into an unintended full-library scan.
• Newly scanned folders remain duplicated and incorrectly marked as unscanned.

backend/endpoints/sockets/scan.py; frontend/src/stores/platforms.ts

Important Files Changed

Filename	Overview
backend/endpoints/sockets/scan.py	Adds slug-based targeting, but rejected explicit selections can fall through to a full-library scan.
backend/utils/platforms.py	Builds local platform schemas and lists filesystem folders without database rows.
backend/endpoints/platform.py	Adds the protected filesystem-platform route before the numeric platform route.
frontend/src/stores/platforms.ts	Combines database and filesystem platforms without reconciling stale entries after scans.
frontend/src/v2/views/Scan.vue	Loads filesystem folders and sends scan selections as filesystem slugs.
frontend/src/v2/components/shared/PlatformSelect.vue	Supports filesystem-slug keys and an opt-in unscanned marker.
frontend/src/services/api/platform.ts	Adds the client call for the filesystem-platform endpoint.

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
backend/endpoints/sockets/scan.py:760
**Rejected Selection Scans Everything**

When a client submits only stale, renamed, or invalid slugs, the preceding filter leaves `selected_slugs` empty and this fallback starts a scan of every filesystem platform. A targeted request must use the scan-all fallback only when the client supplied no platform selection.

```suggestion
    has_selection = bool(platform_ids or platform_fs_slugs)
    platform_list = sorted(selected_slugs if has_selection else fs_platforms)
```

### Issue 2 of 2
frontend/src/stores/platforms.ts:38-41
**Scanned Folders Stay Unscanned**

After a first scan creates a database row, the scan lifecycle refreshes `allPlatforms` but leaves the old entry in `filesystemPlatforms`. This getter then shows the same `fs_slug` twice, including a stale `id: -1` row still marked as unscanned; deduplicate by `fs_slug` while preferring the database entry.

```suggestion
    scannablePlatforms: ({ allPlatforms: all, filesystemPlatforms: fs }) =>
      uniqBy([...all, ...fs], "fs_slug").sort((a, b) =>
        a.display_name.localeCompare(b.display_name),
      ),
```

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