fix(extensions): restore UI extension targets on add + deploy (DEVX-541)

[smukherjee-godaddy]

Problem

UI extension targets were lost end to end:

• application add extension embed/checkout had no --target flag and wrote targets: [] to godaddy.toml.
• collect_extensions() dropped targets entirely.
• deploy_extension() uploaded every extension with target = <extension name> instead of the configured target surface.

So embed/checkout artifacts were uploaded to the wrong target locations, and there was no way to specify where an extension should render.

Fix

Ports the TS target handling:

• add extension embed / checkout — add a required --target (comma-separated, at least one). parse_targets() trims, splits, and drops empty entries; the parsed targets are written to godaddy.toml (previously hard-coded empty). blocks is unchanged (it takes no target).

• collect_extensions() — carry each extension's targets via a small ExtensionDeploy struct instead of discarding them.

• deploy_extension() — bundle once, then upload once per resolved target. resolve_upload_targets() maps:

  • blocks → the fixed "blocks" target,
  • embed/checkout → each configured target, or a single untargeted upload (None) when none are configured.

  The upload's target is now the configured value, not the extension name.

Impact

• Extensions upload to their configured target surfaces.
• Multi-target extensions upload once per target.
• Closes the extension chain: add (--target) → release maps targets (DEVX-540) → deploy uploads per target.

Testing

• Unit tests: parse_targets (trim/split/drop-empty) and resolve_upload_targets (blocks / no-target / multi-target).
• cargo test / cargo clippy --all-targets -- -D warnings / cargo fmt --check — all clean.

Context

Track A ("ship an app"). Pairs with DEVX-540 (release input maps these targets) and DEVX-539 (deploy activation).