fix(installed-apps): prefer Steam client icon and honour the store fallback

[divya0795]

Fixes #166

Two defects in installed-data.js, both affecting the My Games snapshot. They share a file, so they share a PR — no other open PR of mine touches it.

1. Steam client_icon was unreachable

pickImageUrl returns the first non-null candidate in argument order, and getSteamClientIconUrl(...) sat at argument 7, behind six title.* image fields. Any title carrying flat artwork won, so the client icon CDN shape was never selected — the inverse of the rule in AGENTS.md:20 ("must prefer … whenever the matched title/game/version metadata contains a Steam AppID, regardless of install platform").

Moved it to the front. It returns null when no AppID resolves, so non-Steam titles keep falling through to their own metadata with identical results.

The sidebar candidate moved the other way — argument 8 → last. AGENTS.md describes the rendered sidebar DOM as the fallback used only when metadata does not expose the icon, but it was overriding every game.* field.

2. Empty service slice bypassed the store fallback

isRecord({}) is true, so an installedAppsService.installedApps of {} (its state before refreshApps fills it) took the service branch with nothing to match and skipped the store branch below. Now that branch requires a non-empty record. catalog and installedGameVersions are untouched — they already chain their own store fallbacks.

Tests

Three tests added, all driven through the public buildSnapshot / resolveInstalledData contracts. Per web-panel/CLAUDE.md ("test product behavior and public contracts", "keep implementation-only helpers private") I did not export pickImageUrlForTitle to test it directly — the fixtures go through the real snapshot path instead.

Both bug-specific tests were confirmed to fail against the unfixed source before the fix was applied:

× prefers the Steam client icon over the title's own artwork fields
  Expected: "https://api-cdn.wemod.com/steam_community/1245620/client_icon/96.webp"
  Received: "https://cdn.example/subnautica-cover.webp"

× reads installed apps from the store while the service slice is still empty
  Expected: "store"
  Received: "service"

The third test pins the non-Steam path (epic: correlation → title artwork still wins) so the reorder cannot over-correct.

Verification

Run locally on Node 22.23.1 / pnpm 10.17.0, the versions .github/workflows/build.yml pins:

• pnpm test — 37 passed (12 files), up from 34 on master
• pnpm lint — clean at --max-warnings=0
• pnpm typecheck — clean (both typecheck:web and typecheck:bridge)

Note on visible behaviour

This intentionally changes what users see: Steam titles that previously showed a wide catalog cover will now show the 96px client_icon. That is what AGENTS.md mandates, but it is a visual change rather than a silent correctness fix, so flagging it explicitly in case the documented rule no longer reflects your intent — happy to drop that half and keep only the store-fallback fix if so.

I do not have a licensed WeMod install, so I have not exercised this against a live Wand renderer; the evidence above is the unit suite plus the documented invariant.