1137: Fix extraction editor unsaved-changes detection

[katsiarynaby]

Summary

• Fix incorrect "You have unsaved changes" banner when text is reverted, blocks are created then deleted, or edits survive a create/delete cycle
• Compare editor HTML against saved extraction (savedEditorPages) separately from the session baseline (baselinePagesRef), which can drift after block create/delete
• Extend splitHtmlByPage and add editorPageContentMatchesSaved so Tiptap markup and saved hOCR blocks compare correctly
• Refactor useExtractionState helpers for clarity (session vs saved comparison, pending reconciliation pipeline)

Problem

hasChanges could be wrong in several manual flows:

Flow	Expected	Before
Edit text → revert to original	Banner clears	Sometimes stayed
Create block → delete block	Banner clears	OK
Edit → create → delete	Banner stays (text edit remains)	Sometimes cleared
Edit → create → delete → revert text	Banner clears	Sometimes stayed

Root causes:

• Pending pages were reconciled against a drifted session baseline that already included in-progress edits
• Saved extraction uses .ocr_carea[id] while the editor uses data-block-id, so comparisons against originalEditorPages often failed

Solution

• Session baseline — detect edits within the current editor session
• Saved extraction — decide whether content still differs from the server
• On content change: align baseline when editor matches saved, clear pending when fully reverted (unless structural edits exist)
• On deleting the last session-created block: drop pending only when it matches saved content, not the drifted baseline

Test plan

• npm run test:run -- src/features/workspace/ (153 tests)
• Edit block text → revert manually → banner clears
• Create block → delete block → banner clears
• Edit block 1 → create block 2 → delete block 2 → banner stays
• Same as above → revert block 1 text → banner clears