Upload destinations / share targets (webhook v1)

[dimitar-radenkov]

Summary

Add post-capture "share" actions that upload a capture and return a link, starting with configurable webhook/HTTP targets rather than baking in every service.

Why

Sharing friction drives churn and is a high-volume demand theme; ShareX's 80+ destinations and Zight's named integrations are why professionals pick them over free tools (see docs/feature-research.md, Tier 2 / Sharing Pipeline v2). A generic webhook target is a low-commitment first step before any first-party backend.

Scope (v1)

• Configurable upload target (URL, method, headers, field name) in Settings.
• "Upload & copy link" action in the post-capture toolbar/HUD.
• Response template to extract the returned URL and copy it to the clipboard.

Acceptance criteria

• User can configure a custom upload destination.
• Uploading a capture copies the returned link to the clipboard.
• Failures surface a clear error (no silent failure).

Entry points

OverlayViewModel (post-capture actions), UserSettings / SettingsViewModel, a new IShareService.