fix(editor): initialize default page state for new blank notes

[Dronquavious]

Summary

Fixes an issue where non-drawing canvas tools (such as inserting images, creating text boxes, and selecting elements) are unresponsive when opening a new, blank note until an initial pen stroke is drawn.

Root Cause

When opening a new note, readNoteBody(id) returns null. NoteScreen previously left its enginePages state as an empty array ([]). Because InfiniteInkCanvas does not fire onPagesChange during initial mounting, enginePages.length remained 0.

This empty state caused:

• Image insertion (applyInsertedImage) to calculate a target page index of -1 (Math.min(currentPageIndex, 0 - 1)), aborting image placement.
• Text box creation (mutateOverlay) to look up enginePages[0], finding undefined and dropping overlay updates.
• OverlayLayer screen-to-page coordinate mapping to fail.

Once any handwriting occurred, native drawing events triggered ensureSingleTrailingBlankPage(), which fired onPagesChange and synchronized enginePages, unblocking the remaining tools.

Proposed Changes

• In app/note/[id].tsx, updated NoteScreen initialization so that when readNoteBody(id) returns null (for new notes), it falls back to a default 1-page SerializedNotebookData structure.
• enginePages and overlayMap are now populated immediately upon note creation, and the initial page state is passed to canvasRef.current?.loadNotebookData().

Test Plan

• Create a new blank note and immediately insert an image or text box without drawing first.
• Verify existing notes with saved body data continue to load normally.
• Ran npm run typecheck to confirm zero TypeScript errors.