refactor: consolidate SCSS styles and improve code organisation

Eliminates substantial duplication across theme and AMP stylesheets by extracting common patterns into shared modules. Previously, identical styling for entry borders, metadata, timeline pseudo-elements, and pagination appeared in multiple files with slight variations, making updates error-prone and inconsistent.

Introduces a new `_theme-shared.scss` partial containing ~120 lines of common styles, reducing `_theme.scss` from 140 to 10 lines and `_amp.scss` from 230 to 105 lines. Adds `$font-family-base` variable to eliminate four instances of hardcoded font stacks. Creates `_button-base.scss` mixin to standardise button reset patterns used across both app and editor contexts.

Renames `_event.scss` to `_event-item.scss` and `_events.scss` to `_key-events.scss` to clarify their distinct purposes: individual timeline items versus the key events container. This naming better reflects the actual structure and reduces confusion when navigating the codebase.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: GaryJones

assets/amp-rtl.css

@@ -1 +1 @@
-<?php return array('dependencies' => array(), 'version' => 'dab88f5f7ff2cd3e7c90');
+<?php return array('dependencies' => array(), 'version' => '3f962100a8f25a826441');