ci: cache lux_app dialyzer PLTs in full CI

[jeevan6996]

Description

This updates the lux_app job in .github/workflows/ci.yml to cache dialyzer PLTs using restore/save cache actions and to run dialyzer as part of the full CI workflow.

Closes #18
/claim #18

Changes

• replace generic dialyzer cache step with explicit actions/cache/restore@v4 + actions/cache/save@v4
• cache both priv/plts/*.plt and priv/plts/*.hash
• build PLT only on cache miss
• run mix dialyzer --format github in the lux_app job so cached PLTs are actually reused

Testing

• Verified workflow YAML changes and cache key/path consistency with existing lux-app-ci.yml setup.
• Local Elixir CI execution not run in this environment (toolchain/services not available locally).

[jeevan6996]

Friendly ping: this CI dialyzer cache update for is ready for review. I can quickly apply any requested adjustments.

[jeevan6996]

Small correction to my previous ping: this change updates dialyzer caching for the lux_app job in full CI and is ready for review.