Foundation + Color tab fully wired

[bph]

Part of #836. First feature PR after the endpoint lands. Wires the modal's Color tab end-to-end so users can persist palette / gradient / duotone / default-toggle changes to theme.json. Other tabs continue rendering but their Update is a no-op until subsequent PRs.

Depends on #837.

Scope

Files

• src/resolvers.js — add postUpdateThemeSettings(payload) mirroring postUpdateThemeMetadata.
• src/editor-sidebar/edit-theme-settings-modal.js — lift Color-tab state to modal level, wire handleUpdateClick to call the resolver, add loading / success-notice / error-notice / refresh.
• src/plugin-styles.scss — any save-state styling adjustments (loading indicator on the footer button).

Behavior

• Update button enabled when there are pending changes; disabled + spinner during save.
• Success: surface createSuccessNotice, refresh the theme entity record (panels' useEffect resets local state from new server data automatically). Modal stays open by default.
• Error: surface createErrorNotice with the WP_Error message.

State lifting

Modal owns:

• colorSettings (defaults flags + custom flags + link)
• palette / gradients / duotone arrays

Passed to ColorSettingsPanel / PalettePanel / GradientPanel / DuotonePanel as value + onChange. Tabs other than Color keep their existing local state for now.

Acceptance criteria

• Edit any Color sub-panel value, click Update — value persists to theme.json on disk.
• Reload editor — change is reflected.
• Failure (e.g. 500) surfaces an error notice; modal stays open.
• Update button shows loading state during save and is disabled when there are no pending changes.
• Other tabs (Dimensions / Typography / Shadows / Templates) still render and do not error.
• e2e smoke test for the Color save path (if e2e infra is in place per Add basic e2e tests #811).

Out of scope

• Other tabs' persistence (subsequent PRs, see [Tracking] Edit Theme Settings: phased implementation #836).
• WP-CLI command.
• Documentation in readme.txt — fold into this PR if user-visible documentation is desired before further tabs land; otherwise the final per-tab PR.

References

• Tracking issue: [Tracking] Edit Theme Settings: phased implementation #836
• Endpoint sub-issue: REST endpoint + CBT_Theme_Settings_Save service #837
• Pattern mirrored for save UX: metadata-editor-modal.js.