fix(articles): iframe preview + collapsible meta + word count

Replace the auto-escaped `{{ body_html }}` div (showed raw HTML tags)
with a sandboxed `<iframe srcdoc>` pattern matching the email preview.
Both the initial page load and the HTMX live-update endpoint now build
a full `preview_doc` document embedding the public CSS and wrapping
content in `<div class="article-body">` — pixel-perfect against the
live article, admin styles fully isolated.

Also:
- Delete ~65 lines of redundant `.preview-body` custom CSS
- Add "Meta ▾" toolbar toggle to collapse/expand metadata strip
- Add word count footer in the editor pane (updates on input)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Fixed
+- **Article editor preview now renders HTML correctly** — replaced the raw `{{ body_html }}` div (which Jinja2 auto-escaped to literal `<h1>...</h1>` text) with a sandboxed `<iframe srcdoc="...">` pattern. The route builds a full `preview_doc` HTML document embedding the public site stylesheet (`/static/css/output.css`) and wraps content in `<div class="article-body">`, so the preview is pixel-perfect against the live article. The `article_preview` POST endpoint uses the same pattern for HTMX live updates. Removed ~65 lines of redundant `.preview-body` custom CSS from the editor template.
+
 ### Changed
 - **Unified confirm dialog — pure HTMX `hx-confirm` + `<form method="dialog">`** — eliminated the `confirmAction()` JS function and the duplicate `cloneNode` hack. All confirmation prompts now go through a single `showConfirm()` Promise-based function called by the `htmx:confirm` interceptor. The dialog HTML uses `<form method="dialog">` for native close semantics (`returnValue` is `"ok"` or `"cancel"`), removing the need to clone and replace buttons on every invocation. All 12 Padelnomics call sites converted from `onclick=confirmAction(...)` to `hx-boost="true"` + `hx-confirm="..."` on the submit button. Pipeline trigger endpoints updated to treat `HX-Boosted: true` requests as non-HTMX (returning a redirect rather than an inline partial) so boosted form submissions flow through the normal redirect cycle. Same changes applied to BeanFlows and the quart-saas-boilerplate template.
   - `web/src/padelnomics/admin/templates/admin/base_admin.html`: replaced dialog `<div>` with `<form method="dialog">`, replaced `confirmAction()` + inline `htmx:confirm` handler with unified `showConfirm()` + single `htmx:confirm` listener