feat: write migrated assets to theme during themes:migrate

[luis-almeida]

Summary

• The migrations API now returns an assets hash (filename → base64-encoded content) alongside metadata and templates
• Added rewriteAssets to decode and write each returned asset to the theme's assets/ folder
• Existing assets with the same filename are overwritten

Test plan

• Unit tests for rewriteAssets (base64 decoding, multiple file types, empty hash, error handling)
• Unit tests for migrate updated to verify rewriteAssets is called with correct args
• Functional test updated to verify asset file written to disk after migration

[Copilot]

Pull request overview

Adds support for persisting migrated theme assets returned by the theming migrations API by decoding base64 content and writing files into the local theme’s assets/ directory.

Changes:

• Extend themes:migrate to call a new rewriteAssets helper with API-returned assets.
• Add rewriteAssets implementation + unit tests for decoding/writing and error cases.
• Update unit + functional tests to assert assets are written after migration.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/zcli-themes/tests/functional/migrate.test.ts	Verifies a migrated asset file is written to disk and cleans it up after test runs.
packages/zcli-themes/src/lib/rewriteTemplates.ts	Changes template rewrite behavior to throw on write failures instead of ignoring them.
packages/zcli-themes/src/lib/rewriteTemplates.test.ts	Updates tests to assert rewrite failures now throw with the expected message.
packages/zcli-themes/src/lib/rewriteAssets.ts	New helper to mkdir assets/ and write base64-decoded assets to disk.
packages/zcli-themes/src/lib/rewriteAssets.test.ts	Unit coverage for decoding, multiple file types, empty input, and error handling.
packages/zcli-themes/src/lib/migrate.ts	Invokes rewriteAssets with the migrations API response.
packages/zcli-themes/src/lib/migrate.test.ts	Asserts rewriteAssets is called with the expected arguments from the API response.

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[BrunoBFerreira]

Just left two comments. One minor and the other related with the logic we have added here for the construction of the file path. If we want to be more rigid regarding the paths we want to allow, maybe a validation should be added.