feat: Freighter wallet deep-link / QR for mobile contributors who don't have the browser extension

[dotunv]

Description

ContributeModal requires the Freighter browser extension to sign contributions. Mobile users and users without Freighter installed hit a dead end — the modal detects no wallet and shows an error with no actionable alternative.

Proposed Solution

• When Freighter is not detected, show a friendly fallback screen in ContributeModal with two options:
  1. WalletConnect / LOBSTR deep-link — generate a stellar:pay URI that opens a Stellar-compatible mobile wallet app
  2. Custodial contribution — use the existing custodial wallet path (the platform signs on behalf of the user using their stored keys)
• Add a "Get Freighter" link that opens the extension store in a new tab

Acceptance Criteria

• Mobile users or users without Freighter see actionable alternatives, not just an error
• stellar:pay URI is correctly formatted with campaign wallet address, amount, and asset
• Custodial path is clearly labeled as "Contribute without a wallet extension"
• "Get Freighter" link is present and opens the correct store page

[Silver36-ship-it]

I would love to work on this professionally, please assign it to me

[akinboyewaSamson]

@akinboyewaSamson has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi! I’m a full-stack developer with a strong blockchain background. I’d love to handle this issue and can deliver a clean, reliable solution. Kindly assign it to me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @akinboyewaSamson to this issue.

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Congratulations, @akinboyewaSamson! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@akinboyewaSamson's PR #309 was approved and merged by @dotunv.

🏆 @akinboyewaSamson: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (716 total points). Track your full progress on GrantFox.

👏 Great work, @akinboyewaSamson! Keep contributing to Savitura.

[drips-wave]

This issue has been completed by @akinboyewaSamson as part of the Stellar Wave Program's 6th Wave 🥳

😎 @akinboyewaSamson: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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