fix(demos): repair the AOP switch and split the demo into backend and frontend

[lisachenko]

What and why

Bug: with AOP switched off, the demo page crashed halfway through rendering. The source panels called MagicConstantTransformer::resolveFileName(), which needs the aspect kernel, and the kernel isn't booted in that mode. The fatal error cut the page off before the Bootstrap bundle and the switch script were output, so the switch stuck on "Off" and none of the dropdown menu items could be clicked.

Restructure: the backend and frontend are now in separate files:

Part	Files
Front controller	demos/index.php (~45 lines): stores the switch state, runs the selected showcase, renders the template
Backend	Demo\Showcase\{Showcase, ShowcaseCatalog, ShowcaseResult, SourceCode}: the catalog of examples, each with its scenario closure. Only woven classes (Proxy) have their file name resolved back from the cache
Template	demos/templates/page.php: plain HTML, only escaping and loops
Styles / behaviour	demos/assets/demo.css, demos/assets/demo.js

Demo\Highlighter is removed; highlighting is done in the browser with highlight.js.

UI:

• AOP switch: a real switch, posted to the server, which sets the cookie and redirects back, so it works without JavaScript.
• Navigation: the examples are in a sidebar of plain links, which becomes a drawer on mobile. There are no more dropdowns.
• Design: light and dark themes that follow the system setting, an animated gradient background, translucent "glass" panels and a landing page with one card per example.
• Example pages: the output is shown in a terminal-style panel with the run time. The usage code, aspect and class are in source tabs, with syntax highlighting and copy buttons.

CI: the demo smoke test used to grep case '…' lines out of index.php. It now lists the showcases from the catalog and renders every page with AOP on and off. It also checks that each page is rendered all the way to </html>, which catches the original bug: the old code fails this check.

Checklist

• The title follows Conventional Commits (type(scope): summary)
• Tests cover the change (the CI demo smoke test now covers both AOP modes and complete rendering)
• composer check passes (php-cs-fixer clean, phpstan level 10 clean, phpunit 3104 tests OK)
• User-facing or breaking changes are in CHANGELOG.md (not applicable: the demos are not part of the package)

🤖 Generated with Claude Code

https://claude.ai/code/session_01TkXmBX6yR2tyL4TqAzphcm

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.81%. Comparing base (f8e80c1) to head (e94fb36).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #766   +/-   ##
=========================================
  Coverage     97.81%   97.81%           
  Complexity     1841     1841           
=========================================
  Files           104      104           
  Lines          4849     4849           
=========================================
  Hits           4743     4743           
  Misses          106      106           

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