Add webhook self-certification and preflight submission checks

[simonhamp]

Summary

• Developers can self-certify webhook installation when automatic registration fails, with manual setup instructions always visible
• Adds "Retry automatic setup" button alongside self-certification
• Preflight checks (license file, release version, webhook) now gate plugin submission — plugins stay in draft until all required checks pass
• Verifies webhook existence via GitHub API during preflight checks, correcting stale webhook_installed flags
• Adds description validation to submission flow
• Enhances plugin summary card with author name, version, license, min iOS/Android versions, and support channel with appropriate links
• Replaces session flash banners with Flux toasts and adds <flux:toast /> to the dashboard layout
• Scrolls to first validation error on form submission failures
• ReviewPluginRepository now always populates review_checks even when GitHub API calls fail, ensuring the checks panel always renders

Test plan

• 31 tests passing across PluginStatusTransitionsTest, PluginWebhookCertificationTest, and CustomerPluginReviewChecksTest
• Verify webhook self-certification flow works end-to-end
• Verify preflight checks block submission when license/release/webhook missing
• Verify Flux toasts appear correctly for success and error states
• Verify scroll-to-error works on form validation failures
• Verify plugin summary card shows all details with correct links

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