refactor: clean up legacy code in index.php

[akshayurankar48]

Summary

Several legacy code patterns in `index.php` need cleanup:

1. Duplicate enqueue logic (`post_enqueue` / `post_new_enqueue`)

Both methods contain ~30 lines of near-identical script/style enqueue code, differing only in the hook value checked. Should be extracted into a single shared private method.

2. Dead code in `bsf_color_scripts()`

The method checks `if ( 3.5 <= $wp_version )` to decide between `wp-color-picker` and the older `farbtastic`. Since the plugin requires WP 3.7+, the `farbtastic` branch can never execute and should be removed.

3. Bare `die()` in AJAX handlers

`submit_request()` and `submit_color()` both call bare `die()` to terminate. WordPress AJAX handlers should use `wp_die()` for proper interoperability.

4. `esc_html_e()` misused in ternary expressions

`esc_html_e()` echoes and returns `null`, so the ternary condition is always truthy — incorrect output when the operation fails. Should be restructured with `if/else`.

Acceptance Criteria

• `post_enqueue` / `post_new_enqueue` share a single extracted helper
• Dead `farbtastic` branch removed from `bsf_color_scripts()`
• `die()` replaced with `wp_die()` in AJAX handlers
• Ternary `esc_html_e` antipattern fixed in `submit_request()` and `submit_color()`
• PHPCS/PHPStan pass with no new errors