Centralize link key normalization logic to @dub/utils

[ANU-2524]

Currently, the logic for processing and normalizing link keys (handling case-sensitivity, + for inspection, and punycode) is duplicated across several files. This creates maintenance overhead and potential inconsistencies between the redirect middleware and the API.

Files to address:

apps/web/lib/middleware/link.ts
apps/web/lib/api/links/utils/process-key.ts

Proposed Solution:

1. Extract the normalization logic into a pure function in packages/utils.
2. This function should handle:

• punycode encoding/decoding.
• Case normalization based on the domain's caseSensitive setting.
• Stripping the + suffix used for info pages.

3. Update all call sites to use this shared utility.

[linear]

ENG-1705

[ANU-2524]

Hi! I worked on this issue and implemented a fix.

Branch:
https://github.com/ANU-2524/dub/tree/refactor/normalize-link-key-logic

I centralized the link key normalization logic into @dub/utils to reduce duplication and keep behavior consistent across the codebase.

Since PR creation seems restricted for non-collaborators, sharing the branch here for review.