Add notes and support channel fields to plugin submissions

[simonhamp]

Summary

• Adds notes and support channel fields to the plugin submission form
• Both fields appear only after the developer selects a repository
• Support channel accepts an email address or URL, with guidance about URL content
• Notes field suggests sharing links to videos of the plugin working
• Both fields are displayed read-only in the Filament admin dashboard under "Developer Submission Details"
• Neither field is shown on public plugin listings

Changes

• Migration: Adds notes (text) and support_channel (varchar) nullable columns to plugins table
• Livewire component: New notes and supportChannel properties with validation
• Blade view: Two new cards (conditionally shown after repo selection), repo select uses wire:model.live
• Filament resource: Read-only "Developer Submission Details" section with both fields
• Factory: withNotes() and withSupportChannel() states
• Tests: 8 tests covering saving, null handling, conditional visibility, and factory states

Test plan

• Notes are saved when provided during submission
• Support channel (email or URL) is saved when provided
• Both fields store null when left empty
• Fields are hidden until a repository is selected
• Fields appear in Filament admin as read-only
• Factory states work correctly

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