Add custom taxonomy support to Press This panel

[faisalahammad]

Summary

Custom taxonomies registered for the post type now show up in the Press This panel, alongside Categories and Tags. Previously only Category and Tags were ever shown, even if a site had other taxonomies registered.
Fixes #9

Changes

class-wp-press-this-plugin.php

Before:

$categories_tax  = is_object_in_taxonomy( $post_type, 'category' ) ? get_taxonomy( 'category' ) : false;
$tag_tax         = is_object_in_taxonomy( $post_type, 'post_tag' ) ? get_taxonomy( 'post_tag' ) : false;
// ... only category/post_tag data ever built for the panel

After:

$custom_taxonomies_data = $this->get_custom_taxonomies_data( $post_type );
// new method enumerates get_object_taxonomies(), excludes category/post_tag,
// filters to show_ui + assign_terms capability, bootstraps terms for hierarchical taxonomies

Why: Category and Tags keep their own dedicated panels, but every other registered taxonomy is now enumerated and sent to the editor, gated the same way Category/Tags already were (registration + capability).

press-this-plugin.php

Before:

// REST /save only ever accepted 'categories' and 'tags' params

After:

'tax_input' => array( 'type' => 'object', 'default' => array() ),
// save handler validates each taxonomy is registered for the post type, has show_ui,
// and the user can assign_terms, before merging into $post_data['tax_input']

Why: Needed a generic way to save terms for an arbitrary taxonomy, with the same gating applied on save as on read, so a taxonomy hidden from the panel can't be written to by crafting a request by hand.

src/components/TaxonomyPanel.js (new)

New component that renders one panel per custom taxonomy: a checkbox tree for hierarchical taxonomies (same interaction as the existing Category panel), or a token field for flat ones (same interaction as Tags).

src/components/PressThisEditor.js, src/App.js

Wired the new taxonomies data through, added taxonomyTerms state per taxonomy, included it in the save payload.

Testing

Test 1: Hierarchical custom taxonomy

1. Register a hierarchical taxonomy for post (e.g. genre) with some terms.
2. Open Press This.
3. Select a term in the new Genre panel and save as draft.
   Result: post saved with the selected term.

Test 2: Flat custom taxonomy

1. Register a flat taxonomy for post (e.g. rating).
2. Open Press This, type a new term into the Rating panel, save.
   Result: term is created and assigned to the post.

Test 3: Gating

1. Register a taxonomy with show_ui => false, or as a user without assign_terms capability for a taxonomy.
   Result: panel doesn't show, and a hand-crafted REST request with that taxonomy in tax_input is silently ignored (no error, term not saved).

Test 4: Category/Tags unaffected

1. Use Press This as before with no custom taxonomies registered.
   Result: Category and Tags panels behave exactly as before.

Ran the full PHPUnit and Jest suites plus PHPCS/ESLint, all green.