Improve autosave pipeline with queued writes + retry + flush-on-close guard

[clFaster]

Goal

Improve autosave pipeline with queued writes + retry + flush-on-close guard

Type

Technical + Reliability

Effort

M

Impact

High

Evidence

Debounced save every 1s in src/components/ExcalidrawFrame.tsx can overlap with heavy edits and app lifecycle events

First Implementation Step

Add single-flight save queue with last-write-wins and retry backoff