Support Obsidian-style callouts

[lylo]

Obsidian callout syntax was falling through to a plain blockquote with the [!note] marker visible to readers. Reported by a customer publishing a technical blog from Obsidian.

Fixes #1155

What changed

Markdown. Post::Markdown renders > [!note] Title as <aside data-callout="note"> with the title in its own <p data-callout-title>. Obsidian's full type list is supported, with aliases folded onto the type they render as (important renders as tip, caution as warning) and unrecognised types falling back to note, both matching Obsidian. Adjacent callouts separated only by a blank line are split, working around Redcarpet's Markdown.pl quote merging. Nested callouts work.

Editor. Lexical only keeps elements it has a node for, so without editor support a callout was silently flattened to plain paragraphs on first save. A Lexxy extension (app/javascript/editor/) registers callout and title nodes so callouts round-trip, and adds a toolbar picker (colour swatch per type, next to the quote button). The callout node is a Lexical shadow root, so lists, tables and headings inserted inside a callout stay inside it; a node transform keeps the title as the first child and makes block-format commands no-ops on it. Enter on an empty last line, backspace at the title start, and arrow keys at document edges all leave the callout.

Styling. One definition in lexxy-typography.css serves the editor and published blogs. Digest emails get a coloured left border and bold title in mailer.css.

Sanitisers. aside, data-callout and data-callout-title are allowlisted in ActionText and Html::Sanitize, so callouts survive storage, exports and RSS.

Behaviour changes

• Posts published from Obsidian with callouts now render styled callouts instead of quotes with a visible marker.
• The editor toolbar gains a callout button.
• No changes to existing posts; blockquotes without a marker are untouched.