fix(timer): refactor to StateField for proper block decorations

Replace ViewPlugin with StateField to resolve CodeMirror block decoration
error. Use editorInfoField for proper editor access and replace innerHTML
with Obsidian's native DOM APIs.

- Replace ViewPlugin with StateField for block-level decorations
- Use state.field(editorInfoField) to access editor and app instances
- Remove innerHTML usage in favor of createEl/createDiv/createSpan
- Fix "Block decorations may not be specified via plugins" error
- Improve CSS with proper width and box-sizing for block display
- Maintain all existing timer functionality (start/pause/complete)

Author: Quorafind