Design auth screens/states; add previewer for working on them

[jzimdars]

This adds design for the locally served auth screens and a previewer for viewing them in dev make preview-callback.

Supports dark and light mode.

[Copilot]

Pull request overview

Adds styled light/dark OAuth callback screens and a development preview server.

Changes:

• Adds reusable callback templates and HEY branding.
• Integrates status-specific pages into OAuth handling.
• Adds rendering tests and a preview Make target.

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Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Makefile	Adds the callback preview target.
internal/auth/auth.go	Serves status-specific callback pages.
internal/auth/auth_test.go	Tests callback responses and bodies.
internal/auth/callback.go	Embeds and renders callback templates.
internal/auth/callback.html	Defines the responsive light/dark shell.
internal/auth/callback_test.go	Validates rendered callback pages.
internal/auth/callback_preview_test.go	Implements the local preview server.
internal/auth/callback_success.html	Adds the success state.
internal/auth/callback_error.html	Adds the general failure state.
internal/auth/callback_denied.html	Adds the access-denied state.
internal/auth/callback_invalid.html	Adds the invalid-link state.
internal/auth/hey_logo.html	Provides light and dark HEY logos.

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