Bundle and test onboarding URLs bind ":branch" to "new"

[jeffal]

The repo-page route in src/App.tsx matches /bundles/:branch and /tests/:branch against the onboarding URLs /bundles/new and /tests/new, binding :branch = "new". useNavLinks reads that param and emits sibling tab links like /tests/new and /commits/new. Filtering then treats new as a branch name.

Repro:

1. Open a repo with bundle analysis off and JS/TS source.
2. Click Bundles. URL becomes /bundles/new.
3. Click Tests or Commits. URL becomes /tests/new or /commits/new.

[linear]

CCMRG-2214