chore: Remove dfinity utils

[sea-snake]

Remove dfinity utils dependency, given that it's only used for nullish checks and a json replacer/reviver in one place.

Changes

• Remove @dfinity/utils dependency.
• Rewrite all usages of isNullish and nonNullish to plain JS (based on TS value type being either undefined, null or undefined | null).

Tests

All existing tests should pass as-is.

[Copilot]

Pull request overview

This PR removes the direct @dfinity/utils dependency from both the main frontend and the try-ii sub-frontend. The library was only used for isNullish/nonNullish null-check helpers and jsonReplacer/jsonReviver JSON serialization utilities. All usages are replaced with equivalent plain JavaScript comparisons (=== undefined, === null, !== undefined) and an inline re-implementation of jsonReplacer/jsonReviver in writable.store.ts.

Changes:

• Removes @dfinity/utils from package.json and src/try-ii/.../package.json; replaces all isNullish/nonNullish calls with explicit === undefined / === null / !== undefined comparisons across ~50 files.
• Inlines jsonReplacer and jsonReviver (handling BigInt, Principal, Uint8Array) into writable.store.ts and updates the test import accordingly.
• Migrates a few call sites to NonNullable<typeof credential> typed type guards where filter(nonNullish) was previously used as a predicate.

Reviewed changes

Copilot reviewed 73 out of 74 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
package.json / package-lock.json	Removes @dfinity/utils as a direct dependency
src/try-ii/.../package.json	Removes @dfinity/utils from try-ii sub-project
src/frontend/src/lib/stores/writable.store.ts	Inlines jsonReplacer / jsonReviver implementations
src/frontend/src/lib/stores/writable.store.test.ts	Updates jsonReplacer import to local path
src/frontend/src/lib/flows/authFlow.svelte.ts	Replaces nullish checks; accidentally drops configURL from requestConfig
All other .ts / .svelte files	Replaces isNullish/nonNullish with plain JS comparisons

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.