🧹 Fix Unsafe Redux State Mutation in projectsSlice

[sachasayan]

The updateMetaProperty reducer in projectsSlice.ts previously used an unsafe as any cast to assign values to dynamic properties of the activeProject. This bypassed TypeScript's type checking and posed a risk of runtime errors.

I've refactored this to be fully type-safe by:

1. Defining a ProjectMetaUpdate type that links each Project property key with its corresponding value type.
2. Using this type in the updateMetaProperty action payload.
3. implementing a switch statement in the reducer to narrow the types of both the property and value, allowing for safe assignment without casting.
4. Adding a default case with a localized cast to ensure that any new properties added to Project in the future are still handled, preventing the reducer from becoming a restrictive whitelist.
5. Ensuring that projectHasLiveEdits is only updated when a change is actually applied.

This improvement enhances the maintainability and reliability of the project's state management.

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PR created automatically by Jules for task 16382191194992882127 started by @sachasayan

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[sachasayan]

@jules failing ci

[google-labs-jules]

@jules failing ci

I've fixed the regression in projectsSlice.ts where the default case was causing a TypeScript error on the never type. I also took the opportunity to fix a pre-existing TypeScript error in OnboardingPage.tsx that was blocking the build. pnpm typecheck and pnpm test are now passing.

[sachasayan]

@jules rebase

[google-labs-jules]

@jules rebase

I've rebased the branch onto the latest main. I resolved a conflict in projectsSlice.ts and confirmed that typecheck:web and all tests are passing. Note: There are some pre-existing type errors in src/main/index.ts (affecting typecheck:node) that seem to be on the base branch, but they are unrelated to my changes.