Inline polish: :emoji: shortcodes and a strikethrough shortcut

[tosinamuda]

User story
As a writer, I want :shortcode: emoji and a strikethrough shortcut, so that everyday inline formatting doesn't require remembering raw syntax.

Acceptance criteria

• :smile:-style shortcodes render as emoji in rich mode (hide-token style; raw mode shows source); unknown codes stay literal
• A compact bundled shortcode map (no network); typing : + 2 chars offers completions
• Cmd+Shift+X toggles ~~strikethrough~~ (joins Mod-b/i/e in formatCommands); toolbar button included
• Interaction-matrix tests cover caret behavior around the new inline widget

Sub-tasks

• Emoji widget + shortcode map + autocomplete source
• toggleStrikethrough command + keybinding + toolbar
• Tests (formatCommands contract + widget caret rules)