fix: resolve all 8 pre-existing lint errors (closes #29)

Fixed @typescript-eslint/no-explicit-any (4 occurrences):
- FolderEditModal.tsx: proper error type checking instead of any
- StepForm.tsx: explicit union type for visibility select
- StepLibraryBrowser.tsx: explicit union types for stepType and sortBy selects

Fixed react-hooks/set-state-in-effect (1 occurrence):
- NodeEditorModal.tsx: replaced useEffect with direct state comparison

Fixed @typescript-eslint/no-unused-vars (3 occurrences):
- NodeEditorModal.tsx: removed unused useEffect import
- NodeEditorModal.tsx: added eslint-disable for intentionally destructured children
- usePermissions.ts: removed unused _tree parameter from canDeleteTree
- TreeLibraryPage.tsx: updated canDeleteTree call site

Fixed @typescript-eslint/no-empty-object-type (1 occurrence):
- types/step.ts: changed empty interface to type alias

Verification:
- npm run lint: 0 errors (9 warnings are intentional exhaustive-deps)
- npm run build: succeeds
- TypeScript compilation: passes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/step-library/StepLibraryBrowser.tsx

@@ -164,7 +164,7 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
           <select
             aria-label="Filter by step type"
             value={selectedStepType || ''}
-            onChange={(e) => setSelectedStepType((e.target.value as any) || undefined)}
+            onChange={(e) => setSelectedStepType((e.target.value as 'decision' | 'action' | 'solution') || undefined)}
             className="rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
           >
             <option value="">All Types</option>
@@ -190,7 +190,7 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
           <select
             aria-label="Sort steps by"
             value={sortBy}
-            onChange={(e) => setSortBy(e.target.value as any)}
+            onChange={(e) => setSortBy(e.target.value as 'recent' | 'popular' | 'highest_rated' | 'most_used')}
             className="rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
           >
             <option value="recent">Most Recent</option>