Rename ProjectSparks references to ProjectRainbows

[kabeier]

Motivation

• The repository was rebranded from "ProjectSparks" to "ProjectRainbows" and all user-facing text, tests, and packaging metadata must reflect the new name to avoid confusion and keep tooling consistent.

Description

• Replaced the home page heading in pages/index.tsx from ProjectSparks to ProjectRainbows.
• Updated unit and e2e assertions in src/App.test.tsx and e2e/home.spec.ts to expect projectrainbows instead of projectsparks.
• Updated package identifiers in package.json and the root package-lock.json from projectsparks to projectrainbows.
• Updated Docker and documentation references in docker-compose.yml and README.md to use the projectrainbows-web image name.

Testing

• Ran unit tests with npm test -- --watch=false, which passed (all tests green).
• Verified there are no remaining legacy references with rg -n "ProjectSparks|projectsparks", which returned no matches.

---

Codex Task