feat(auth): theme and customize OAuth callback pages

[danielgtaylor]

Summary

• Restore animated OAuth authorization-code callback success and failure pages from v1.
• Use the active Restish theme's status_2xx and status_error colors for the default callback backgrounds.
• Add optional callback_success_html and callback_error_html auth params for branded callback pages; error templates support $ERROR, $TITLE, and $DETAILS with escaped substitutions.
• Keep callback HTML params local to Restish so they are not forwarded to OAuth authorization, device authorization, or token endpoints.
• Document the themed defaults and custom HTML params in the OAuth guide/reference.

Credit: adapts the custom callback HTML idea from #330 by @davidlwg for the v2 OAuth implementation.

Tests

• env GOCACHE=/tmp/restish-gocache go test ./internal/auth -count=1
• env GOCACHE=/tmp/restish-gocache go test ./...

[Copilot]

Pull request overview

Restores branded, animated OAuth authorization-code browser callback pages and adds per-profile customization, integrating with Restish's active theme for default colors.

Changes:

• Adds animated success/failure callback pages with status_2xx/status_error theme-driven background colors and HTML-escaped title/detail.
• Adds callback_success_html / callback_error_html auth params (with $ERROR/$TITLE/$DETAILS escaped substitution) reserved from authorize/token/device endpoints.
• Documents the new behavior in the OAuth guide and auth reference, and adds tests for theme color access, callback rendering, and non-forwarding of HTML params.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/auth/oauth_authcode.go	Adds callback color/HTML fields, themed/animated callback pages, template rendering, and reserves new params from authorize URL.
internal/auth/oauth_authcode_test.go	Updates existing callback assertions and adds tests for escaping, color fallback, custom HTML fields/params, and non-forwarding.
internal/auth/oauth_common.go	Reserves callback_success_html/callback_error_html from token-endpoint extra params.
internal/auth/oauth_common_test.go	Verifies new HTML params aren't forwarded to token endpoint.
internal/auth/oauth_device_code.go	Reserves new HTML params from device-authorization extras.
internal/cli/auth.go	Wires active theme colors into the authorization-code handler.
internal/cli/auth_internal_test.go	Tests theme-color wiring into the handler.
internal/output/style.go	Adds ThemeTokenColor to expose token foreground colors.
internal/output/style_test.go	Tests ThemeTokenColor for known/unknown tokens.
site/content/en/docs/guides/oauth.md	Documents callback HTML customization with example.
site/content/en/docs/reference/auth.md	Lists new optional callback HTML params for oauth-authorization-code.

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