fix(chatgpt): support copying conversations inside projects

[JGSnapp]

Description

Copying a ChatGPT conversation that belongs to a Project failed with a CtxPort · Copy failed — Unknown error toast.

Project conversations live at https://chatgpt.com/g/g-p-<project>/c/<id>, but CONVERSATION_PATTERN only matched /c/<id> directly after the host, so extract() threw E-PARSE-001 ("Not a ChatGPT conversation page").

Changes:

• core-plugins: the ChatGPT conversation pattern now accepts an optional /g/<project>/ prefix. Regular /c/<id> URLs are unaffected.
• browser-extension: useCopyConversation showed "Unknown error" for every plugin failure, because plugins throw plain AppError objects (from createAppError), not Error instances. The toast now shows the AppError detail/message.
• Tests: added chatgpt/__tests__/plugin.test.ts, which covers a regular conversation, a project conversation, and a non-conversation project page.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (not needed)
• My changes generate no new warnings
• Any dependent changes have been merged and published (none)

Testing

• vitest run in packages/core-plugins: 14/14 passed
• pnpm typecheck in apps/browser-extension: passed
• Manually: built the Chrome MV3 extension and copied a conversation inside a ChatGPT project successfully

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