Track draft ID in composer state machine

Adds draftId to ComposerState so that editing an existing draft and
saving it again updates the draft rather than creating a new one.

- Add draftId?: string to ComposerState type
- Set draftId when restoring from draft via restore_from_draft action
- Pass existingDraftId to save functions from composerState.draftId
- Add PageX icon for empty drafts state

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/icons/pageX_stroke2_corner0_rounded_large.svg

@@ -0,0 +1 @@
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