Hero variable system, CSS fallbacks, header cleanup, remove mobile overrides

- Hero card fully variable-driven: --hero-card-bg/color/overlay/border-radius/padding/max-width
- Alternative hero variant: --hero-alt-card-* for secondary theme
- Overlay bg-position and bg-size now customizable via variables
- Mobile breakpoint: photo bg hidden, hero full-bleed on small screens
- Added fallback values to 1365 var() calls in template.css
- .btn --btn-bg changed from transparent to var(--body-bg) in dark/light themes
- .offcanvas-header .btn-close gets background-color from overlay
- Stripped file info headers (@package/DEFGROUP/VERSION) from 48 files, kept copyright/SPDX
- Synced all theme variables across standard and custom templates
- Removed 26 mobile.php layout overrides
- Added *.py to .gitignore, card-border-radius fallback

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/index.php

@@ -514,7 +514,7 @@ $wa->useScript('user.js');         // js/user.js
 <?php if ($this->countModules('drawer-left', true)) : ?>
 <!-- Left Offcanvas Drawer -->
 <aside class="offcanvas offcanvas-start" tabindex="-1" id="drawer-left">
-  <div class="offcanvas-header justify-content-start">
+  <div class="offcanvas-header justify-content-end">
 	<button type="button" class="btn-close text-reset" data-bs-dismiss="offcanvas" aria-label="<?php echo Text::_('JLIB_HTML_BEHAVIOR_CLOSE'); ?>"><span class="fa fa-close"></span></button>
   </div>
   <div class="offcanvas-body">
@@ -526,7 +526,7 @@ $wa->useScript('user.js');         // js/user.js
 <?php if ($this->countModules('drawer-right', true)) : ?>
 <!-- Right Offcanvas Drawer -->
 <aside class="offcanvas offcanvas-end" tabindex="-1" id="drawer-right">
-  <div class="offcanvas-header justify-content-end">
+  <div class="offcanvas-header justify-content-start">
 	<button type="button" class="btn-close text-reset" data-bs-dismiss="offcanvas" aria-label="<?php echo Text::_('JLIB_HTML_BEHAVIOR_CLOSE'); ?>"><span class="fa fa-close"></span></button>
   </div>
   <div class="offcanvas-body">