feat: add GitHub Sponsors dashboard link (#50)

[Pavankalyan32]

Addresses #50 — adds a 'Manage Sponsorship' link/button so sponsors can easily access their GitHub Sponsors dashboard to manage payment methods, tier, etc.

Changes

• Navbar: Added 'Manage Sponsorship' link in the authenticated user dropdown (next to Log Out)
• Sponsor page: Added 'Manage Sponsorship' button alongside existing 'Sponsor elementary' and 'Visit elementary.io' buttons
• Both link to (opens in new tab with )

Testing

• All CI checks pass locally: ✅, ✅, ✅, ✅
• Changes follow existing code patterns (Vue 3, Nuxt 3, Pinia, TypeScript)
• No new dependencies added

Fixes #50

[lewisgoddard]

The https://github.com/sponsors/elementary/dashboard link 404s if you are not a sponsor, so maybe we should link to https://github.com/sponsors/elementary instead. This is how it looks:

@danirabbit Code is fine, thoughts on the link?

[Pavankalyan32]

💡 Suggestion (non-blocking) — updated after lewisgoddard's screenshots: this button is visible to all visitors, and the /dashboard URL 404s for anyone who isn't a sponsor, so non-sponsors get a dead end rather than an empty state. Fixed in the latest commit: both links now point to the public https://github.com/sponsors/elementary page (works for everyone; sponsors still reach their manage options from there). Gating the navbar variant behind the existing useAuthStore.loggedIn remains an option if you want a sponsor-only affordance later.

[Pavankalyan32]

💡 Suggestion (non-blocking): the PR description markdown got mangled — "Both link to (opens in new tab with )" and "All CI checks pass locally: ✅, ✅, ✅, ✅" are missing the actual URLs and check names, so the rendered description doesn't show where the links point or which checks ran. A quick edit to the description body would make it read properly. The code itself is clean — nice, small PR.

[Pavankalyan32]

Thanks @lewisgoddard — good catch, I can reproduce that. I've pushed a fix (572d9cd): both links now point to the public https://github.com/sponsors/elementary page, which serves everyone and shows manage options for existing sponsors. Appreciate the screenshots.