refactor(op-log): drop dead DocumentMode action type

DOCUMENT_MODE_UPDATE_BLOCKS_DELTA and its 'DU' op-log code were added by
the legacy Angular document-mode feature and orphaned when that feature
was removed (de5e399ab). Both were added and removed within this
unreleased branch, so no shipped op-log can contain a 'DU' entry — the
"kept for historical op-log decoding" comment did not apply. Nothing
references the action type; decodeActionType already falls back for
unknown codes.

src/app/core/persistence/operation-log/compact/action-type-codes.ts

@@ -50,10 +50,6 @@ export const ACTION_TYPE_TO_CODE: Record<ActionType, string> = {
   // GlobalConfig actions (C)
   [ActionType.GLOBAL_CONFIG_UPDATE_SECTION]: 'CU',
 
-  // DocumentMode actions (D) — feature removed, kept so historical op-log
-  // entries decode to a stable action type rather than the raw 'DU' code.
-  [ActionType.DOCUMENT_MODE_UPDATE_BLOCKS_DELTA]: 'DU',
-
   // Metric actions (E)
   [ActionType.METRIC_ADD]: 'EA',
   [ActionType.METRIC_DELETE]: 'ED',

src/app/op-log/core/action-types.enum.ts

@@ -104,10 +104,6 @@ export enum ActionType {
   NOTE_UPDATE_ORDER = '[Note] Update Note Order',
   NOTE_MOVE_TO_PROJECT = '[Note] Move to other project',
 
-  // DocumentMode actions — feature removed, enum entry kept so historical
-  // op-log entries decode to a stable action type identifier.
-  DOCUMENT_MODE_UPDATE_BLOCKS_DELTA = '[DocumentMode] Update Document Blocks Delta',
-
   // Project actions (P)
   PROJECT_ADD = '[Project] Add Project',
   PROJECT_UPDATE = '[Project] Update Project',