fix: address code review feedback for excalidraw widget

[AlexKlim]

Fixes from CodeRabbit review on upstream PR wavetermdev#3479:

• Replay persisted pushscene events for file-backed blocks too; scenes carry a pushid marker so a replayed old push cannot clobber newer file content
• Discard stale async Mermaid conversions via a push sequence counter
• Keep files from native Excalidraw JSON pushes so image elements resolve
• Track a change revision for autosave so background-only edits are not falsely marked saved