feature: add autosave settings and save-status UX

[chaoxu]

Summary

Add user-facing settings and status surfaces for autosave, save failures, recovery, and conflicts.

Scope

• Settings for autosave mode and delay.
• Visible save status for the current document.
• Clear error state when a save fails.
• Recovery/conflict entry points so users can understand what happened.
• Avoid noisy chrome during normal typing.

Why

A live-save system is not just backend plumbing. Users need to know:

• whether the file is saved
• whether a save is pending
• whether a save failed
• whether recovered/conflicting content exists

Without this, autosave feels opaque and unsafe even if the implementation is correct.