refactor: clean up legacy code in index.php

[akshayurankar48]

Summary

Closes #235

Cleans up four legacy code patterns in index.php:

• Extracted duplicate enqueue logic — post_enqueue() and post_new_enqueue() shared ~30 lines of identical code. Both now delegate to a new private enqueue_post_editor_scripts() helper, eliminating the duplication.
• Removed dead farbtastic branch — bsf_color_scripts() checked if ( 3.5 <= $wp_version ) to choose between wp-color-picker and the legacy farbtastic. The plugin requires WP 3.7+, so the farbtastic branch was unreachable dead code.
• Replaced die() with wp_die() — submit_request() and submit_color() both used bare die() to terminate AJAX responses. Replaced with wp_die() per WordPress standards.
• Fixed esc_html_e() in ternary — echo $result ? esc_html_e(...) : esc_html_e(...) was always truthy because esc_html_e() returns null. Restructured as if/else so failure messages are actually shown.

Test plan

• Edit an existing post — confirm schema meta box scripts/styles load correctly
• Create a new post — confirm schema meta box scripts/styles load correctly
• Submit the support request form — confirm "Thank you!" / "Something went wrong!" messages display correctly
• Save colour settings from the admin dashboard — confirm "Settings saved!" / error message displays correctly
• Confirm no PHP syntax errors: php -l index.php

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