fix: prevent Ctrl+Z from reverting to default content after note load

[j4rviscmd]

Summary

• Suppress undo history recording during programmatic note content loading (replaceBlocks / backfillImageCaptions)
• Extract duplicated image-caption fallback logic into a shared getImageCaptionFallback helper

Problem

After opening a note, pressing Ctrl+Z would revert the editor content back to the default "Welcome to Scripta" blocks. This happened because replaceBlocks dispatched a ProseMirror transaction that prosemirror-history recorded as an undoable step.

Solution

Added withSuppressedHistory(view, fn) which temporarily patches view.dispatch to attach addToHistory: false metadata on every transaction during the callback. The original dispatch is always restored via try/finally.

Test plan

• Open an existing note and press Ctrl+Z — should not revert to default content
• Open an existing note, type text, then press Ctrl+Z — typed text should be undone
• Create a new note (no noteId) and verify undo works normally

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