fix: Use correct ProseMirror mark types (strong/em)

Outline schema uses:
- "strong" not "bold"
- "em" not "italic"

Error was: "RangeError: There is no mark type bold in this schema"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 All notable changes to this project will be documented in this file.
 
+## [1.3.15] - 2026-01-31
+
+### Fixed
+
+- **ProseMirror Mark Types:** Fixed mark type names to match Outline schema
+  - `bold` → `strong`
+  - `italic` → `em`
+  - Error was: "RangeError: There is no mark type bold in this schema"
+
 ## [1.3.14] - 2026-01-31
 
 ### Added