feat: replace save toast notifications with header status indicator

[j4rviscmd]

Summary

• Replace intrusive toast.success notifications ("Saved", "Note created") with a subtle header-based status indicator
• The indicator shows a pulsing dot while saving, a checkmark on success (auto-fades after 3s), and an error icon + text on failure
• Error toasts (toast.error) are preserved for critical feedback that requires user attention
• SaveStatus type is exported via the feature's public API (features/editor/index.ts)

Changes

File	Change
useAutoSave.ts	Added SaveStatus state + type, replaced toast.success with setSaveStatus, consolidated save success callback
Editor.tsx	Added onStatusChange prop to propagate save status to parent
App.tsx	Added SaveStatusIndicator in header next to ModeToggle
SaveStatusIndicator.tsx	New component with pulsing dot (saving), checkmark (saved), error icon (error)
features/editor/index.ts	Exported SaveStatus type via public API

Test plan

• Type a note and verify a pulsing dot appears while saving
• Verify the checkmark appears after save and fades out after ~3 seconds
• Verify the checkmark stays visible even when typing resumes before the 3s timer expires
• Verify error state shows "Save failed" text with warning icon
• Verify toast.error still appears for save failures
• Verify no toast appears on successful save or note creation
• Test in both light and dark themes

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