fix(platforms): hide empty platforms behind a toolbar filter

[gantoine]

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

#3945 switched the Platforms index from filledPlatforms to allPlatforms so empty (0-game) platforms would be reachable and therefore deletable (#3902). The side effect: every leftover platform row now renders in the grid. Users who keep large exclude.platforms lists (excluded folders leave behind 0-game rows) went from a clean library to hundreds of stubs.

This keeps both properties:

• Empty platforms are hidden by default again.
• A new segmented toolbar cluster (Only platforms with games / Include empty platforms) reveals them on demand, so an empty platform can still be opened and removed through the existing Settings → Delete platform flow.

Details:

• The filter is URL-synced as ?show=all, omitted at the default, matching the ?kind= / ?vis= pattern on the Collections index and the "active filters live in the URL" rule.
• Header count and search run over the visible set.
• The "no platforms found, run a scan" empty state now keys off allPlatforms, so it doesn't misfire when the filter is what's hiding everything. A library whose platforms are all empty gets a message pointing at the filter instead.
• 4 new platform.json keys, translated across all 18 locales.

Note for context on the issue report: the reporter's exclude.platforms list was not wiped by the upgrade. _update_config_file rewrites the full list and there's no truncation path. Those platforms were never hidden by the exclusions themselves, only by the rom_count > 0 filter that #3945 removed.

Fixes #3979

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

Verification:

• npm run test — 40 files, 580 tests pass
• npm run typecheck — clean
• trunk fmt && trunk check — no issues
• python3 src/locales/check_i18n_locales.py and check_i18n_sorted.py — both pass

Not verified in a browser (developed in a worktree without a running app).

AI assistance disclosure

This change was written with AI assistance (Claude Code). I directed the investigation and reviewed the result; the implementation, translations, and tests were AI-generated.

Screenshots (if applicable)

n/a

🤖 Generated with Claude Code

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[greptile-apps]

Greptile Summary

Adds a URL-synchronized platform visibility filter that hides empty platforms by default while keeping them accessible on demand.

• Filters platform counts, search results, grouped views, and list/grid rendering against the selected visibility mode.
• Distinguishes a genuinely empty library from platforms hidden by the default filter.
• Adds unit coverage for default filtering, ?show=all, and the all-empty state.
• Adds the four required strings across all supported locales.

Confidence Score: 3/5

The concurrent URL update path needs to be fixed before merging because toolbar interactions can discard each other's active query state.

The new visibility watcher independently snapshots route.query and calls router.replace, while existing search and view-mode watchers do the same; concurrent updates therefore overwrite one another and reset an active toolbar filter.

Files Needing Attention: frontend/src/v2/views/PlatformsIndex.vue

Prompt To Fix All With AI

### Issue 1
frontend/src/v2/views/PlatformsIndex.vue:89
**Concurrent query updates overwrite filters**

When the platform segment and another URL-backed toolbar control update in the same reactive flush, each watcher copies the same stale `route.query` into a separate `router.replace`, so the later navigation drops the other control's parameter and resets its visible state.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

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