fix: keep top-level dashboard nav highlighted on nested routes (#220)

[tdlxgpp]

Fixes #220

Changes

• Added isNavItemActive(pathname, href) in DashboardShell.tsx.
• Dashboard overview links (/dashboard/*) keep exact matching.
• Top-level sections (/issues, /milestones) stay highlighted on nested child routes such as /issues/abc123.
• Added focused unit tests covering exact routes, nested routes, unrelated sections, and dashboard exact matching.

Verification

• npm test -- --runInBand: 12 suites / 105 tests passed.
• npm run lint: passed.
• NEXT_PUBLIC_API_URL=http://localhost:4000/api NEXT_PUBLIC_STELLAR_NETWORK=TESTNET npm run build: passed.
• Working tree only contains the source change and new test file.

AI Usage

• Yes, used AI assistance.
• I reviewed every line and understand the purpose of the helper and tests.
• The change is kept to the shared dashboard shell only; no global or out-of-scope modifications.

[vercel]

@tdlxgpp is attempting to deploy a commit to the chonilius' projects Team on Vercel.

A member of the Team first needs to authorize it.

[tdlxgpp]

Hi @chonilius, I opened this PR for #220. The Vercel deployment check is waiting for a member of the Vercel team to authorize it. Could you authorize it when you have a moment? Happy to make any changes after review.

[tdlxgpp]

恭喜 @miraclesonly，PR #323 已合并并完成 #220。我这边较早提交的 PR #302 现在让位关闭，不占用维护者 review 资源。祝项目顺利！