feat: add shared app navigation (#1330)

[mittalsonal]

Summary

• add a shared sticky responsive navbar at the app-shell level for both signed-out and signed-in users
• expose dashboard section anchors for Streaks, Pull Requests, and Goals so the new nav links go to real sections
• wire landing page feature/open-source section anchors into the shared navigation

Testing

• git diff --check
• npm run type-check (blocked locally because this checkout does not currently have a runnable TypeScript compiler installed: tsc is not available in PATH and the project dependencies are not installed on this machine)

[vercel]

@mittalsonal is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[mittalsonal]

Maintainer label request: if this looks good, please add the GSSoC labels including gssoc:approved and the appropriate difficulty/type labels so it counts correctly on the platform. Thanks!

[mittalsonal]

Verification note:\n- git diff --check\n\nI also attempted local type-check verification, but this checkout does not currently have a runnable TypeScript compiler installed:
pm run type-check fails because sc is unavailable and the project dependencies are not installed on this machine. I kept the PR scope narrow and avoided overstating verification.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus