feat(frontend): add Twitter/X and LinkedIn share buttons (#836)

[dumbdevss]

Summary

• Add X (Twitter) and LinkedIn share buttons to the bounty detail page header
• Pre-fill share URLs with the bounty title, reward amount, and detail page permalink
• Open share dialogs in a new tab without navigating away from the app
• Include responsive styling so the share button group wraps cleanly on mobile viewports

Test plan

• Unit tests for share URL builders (shareLinks.test.ts)
• Unit tests verifying social share links render with correct prefilled URLs (BountyDetailPage.test.tsx)
• Manually open a bounty detail page and click the X button — confirm the intent dialog includes title, amount, and link
• Manually click the LinkedIn button — confirm the share dialog opens with the bounty permalink
• Resize to a mobile viewport and confirm share buttons remain visible and usable

Closes #836

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