feat: Add custom step continuation flow with save/use/branch options

Custom steps during tree navigation now support a complete workflow:
- PostStepActionModal: Save for Later / Use Now / Both options
- ContinuationModal: Pick descendant nodes or build custom branch
- ForkTreeModal: Save modified tree as personal copy at completion
- Custom steps are recorded in decisions array for export
- Fix popular-tags API endpoint URL mismatch
- Add aria-labels for accessibility on select/button elements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -149,6 +149,7 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
         <div className="grid grid-cols-2 gap-2 sm:grid-cols-4">
           {/* Category Filter */}
           <select
+            aria-label="Filter by category"
             value={selectedCategoryId || ''}
             onChange={(e) => setSelectedCategoryId(e.target.value || undefined)}
             className="rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
@@ -161,6 +162,7 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
 
           {/* Type Filter */}
           <select
+            aria-label="Filter by step type"
             value={selectedStepType || ''}
             onChange={(e) => setSelectedStepType((e.target.value as any) || undefined)}
             className="rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
@@ -173,6 +175,7 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
 
           {/* Min Rating Filter */}
           <select
+            aria-label="Filter by minimum rating"
             value={minRating?.toString() || ''}
             onChange={(e) => setMinRating(e.target.value ? Number(e.target.value) : undefined)}
             className="rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"
@@ -185,6 +188,7 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
 
           {/* Sort By */}
           <select
+            aria-label="Sort steps by"
             value={sortBy}
             onChange={(e) => setSortBy(e.target.value as any)}
             className="rounded-md border border-input bg-background px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ring"