fix(ci): skip mobile workspace patches when mobile node_modules is absent

[dylanjeffers]

Web Build on main fails on a node_modules cache hit:

Error: Patch file found for package lottie-react-native which is not present at node_modules/lottie-react-native

The Lottie patch from #14652 lives in packages/mobile/workspace-patches because npm nests that package under packages/mobile/node_modules. web.yml's cache doesn't include that folder, so on a cache hit the package is missing and --error-on-fail exits. The mobile workflow caches the folder, so mobile builds are unaffected.

The step now runs only when packages/mobile/node_modules exists. Full installs still apply the patch.

🤖 Generated with Claude Code

[changeset-bot]

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Latest commit: 6beca27

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[dylanjeffers]

Verified locally by reproducing a web cache hit (root and web workspace node_modules, no packages/mobile/node_modules): the old script exits 1 with the same lottie-react-native error as CI, and this script exits 0. Web CI doesn't run on scripts/ changes.