feat: Add Payjoin V2 overview page

[ValeraFinebits]

This PR redesigns the Payjoin V2 overview page to better match the rest of the BTCPay UI and make merchant-side availability easier to understand. Refs #35

Changes

• make the overview page current-store aware
• restrict overview access and settings navigation to CanViewStoreSettings for the selected store
• replace the old template/debug-style content with a store-focused Payjoin V2 status overview
• remove the Plugin activity section and the overview dependency on persisted debug data

[Copilot]

Pull request overview

This PR replaces the old plugin/debug overview with a current-store-aware Payjoin V2 status page and restricts navigation/access to users who can view the selected store settings.

Changes:

• Adds a new Payjoin V2 overview controller/view with store configuration and readiness details.
• Updates header navigation to point at the new store-aware overview page.
• Removes the old template/debug plugin page and its persisted debug-data service registration.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
BTCPayServer.Plugins.Payjoin/Views/UIPlugin/Index.cshtml	Removes the old persisted debug-data plugin page.
BTCPayServer.Plugins.Payjoin/Views/UIPayjoinOverview/Index.cshtml	Adds the new Payjoin V2 overview UI.
BTCPayServer.Plugins.Payjoin/Views/Shared/PayjoinHeaderNav.cshtml	Updates top navigation to show Payjoin V2 for the selected store.
BTCPayServer.Plugins.Payjoin/Views/PayjoinNavPages.cs	Renames the nav enum for the new Payjoin overview.
BTCPayServer.Plugins.Payjoin/Services/PayjoinPluginService.cs	Removes the old debug-data service.
BTCPayServer.Plugins.Payjoin/Plugin.cs	Stops registering the removed debug-data service.
BTCPayServer.Plugins.Payjoin/Controllers/UIPluginController.cs	Removes the old template/debug controller.
BTCPayServer.Plugins.Payjoin/Controllers/UIPayjoinOverviewController.cs	Adds the new store-aware overview controller and status model.

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