refactor(ios): extract row detail logic into RowDetailViewModel

[datlechin]

Summary

P1 #5 step 3 of 3: lift business logic out of RowDetailView (514 LOC) into a new RowDetailViewModel. The View drops to 360 LOC and now owns only sheet state (fkPreviewItem, showShareSheet, shareText) and haptic triggers; the ViewModel owns row data, edit lifecycle, save, and lazy cell loading.

What moves into the ViewModel:

• All row state: rows, currentIndex, isEditing, editedValues, loadingCell, fullValueOverrides, isSaving, operationError, showSaveSuccess
• Domain context (let constants): columns, columnDetails, foreignKeys, table, session, databaseType, safeModeLevel, onSaved, loadFullValueProvider
• Methods: startEditing(), cancelEditing(), saveChanges() -> Bool, loadFullValue(ref:cellIndex:), setEditedValue(_:at:), toggleNull(at:), row(at:), cells(at:), columnDetail(for:), isPrimaryKey(at:), hasOverride(forRow:cellIndex:)
• Computed properties: isView, canEdit, supportsLazyLoading, currentRow, currentRowCells
• Auto-dismiss success toast logic (the 2-second Task.sleep + withAnimation cancellation) lives in the VM as scheduleSuccessDismiss(), with dismissSuccessTask cancelled in deinit

What stays in the View:

• fkPreviewItem, showShareSheet, shareText (UI sheet state)
• hapticSuccess, hapticError, hapticSelection (sensory feedback triggers)
• Body composition, toolbar buttons, share menu, row content rendering, editable field rendering
• The operation-error alert binds to viewModel.operationError != nil via a derived Binding<Bool> (no parallel @State flag)

Test plan

• Open a row from DataBrowser: paged TabView swipe between rows updates currentIndex, selection haptic fires
• Tap "Edit" on a row that has a primary key column: editable mode shows TextFields for non-PK fields, PK fields display as read-only with an orange key
• Toggle NULL on a field: text disappears, "NULL" capsule turns accent. Toggle off: empty text returns
• Save with changes: SQL UPDATE runs, success toast appears for 2 seconds, success haptic, parent's onSaved fires, list reloads
• Save with no changes: just exits edit mode (no SQL fired, no error)
• Save error (e.g. constraint violation): operation error alert shows, error haptic, edit mode stays
• Tap "Cancel" while editing: edits discarded, exits edit mode
• BLOB column with cell.isLoadable: "Load full value" button fires, progress shows on that cell, value populates
• FK column tap: opens FKPreviewView sheet
• Share / Copy via toolbar menu, all formats