feat: add mass edit feature for backlog kanban cards

Add ability to select multiple backlog features and edit their configuration
in bulk. Selection is limited to backlog column features in the current
branch/worktree only.

Changes:
- Add selection mode toggle in board controls
- Add checkbox selection on kanban cards (backlog only)
- Disable drag and drop during selection mode
- Hide action buttons during selection mode
- Add floating selection action bar with Edit/Clear/Select All
- Add mass edit dialog with all configuration options in single scroll view
- Add server endpoint for bulk feature updates

apps/ui/src/components/views/board-view/hooks/index.ts

@@ -7,3 +7,4 @@ export { useBoardEffects } from './use-board-effects';
 export { useBoardBackground } from './use-board-background';
 export { useBoardPersistence } from './use-board-persistence';
 export { useFollowUpState } from './use-follow-up-state';
+export { useSelectionMode } from './use-selection-mode';

apps/ui/src/components/views/board-view/hooks/use-selection-mode.ts

@@ -0,0 +1,82 @@
+import { useState, useCallback, useEffect } from 'react';
+
+interface UseSelectionModeReturn {
+  isSelectionMode: boolean;
+  selectedFeatureIds: Set<string>;
+  selectedCount: number;
+  toggleSelectionMode: () => void;
+  toggleFeatureSelection: (featureId: string) => void;
+  selectAll: (featureIds: string[]) => void;
+  clearSelection: () => void;
+  isFeatureSelected: (featureId: string) => boolean;
+  exitSelectionMode: () => void;
+}
+
+export function useSelectionMode(): UseSelectionModeReturn {
+  const [isSelectionMode, setIsSelectionMode] = useState(false);
+  const [selectedFeatureIds, setSelectedFeatureIds] = useState<Set<string>>(new Set());
+
+  const toggleSelectionMode = useCallback(() => {
+    setIsSelectionMode((prev) => {
+      if (prev) {
+        // Exiting selection mode - clear selection
+        setSelectedFeatureIds(new Set());
+      }
+      return !prev;
+    });
+  }, []);
+
+  const exitSelectionMode = useCallback(() => {
+    setIsSelectionMode(false);
+    setSelectedFeatureIds(new Set());
+  }, []);
+
+  const toggleFeatureSelection = useCallback((featureId: string) => {
+    setSelectedFeatureIds((prev) => {
+      const next = new Set(prev);
+      if (next.has(featureId)) {
+        next.delete(featureId);
+      } else {
+        next.add(featureId);
+      }
+      return next;
+    });
+  }, []);
+
+  const selectAll = useCallback((featureIds: string[]) => {
+    setSelectedFeatureIds(new Set(featureIds));
+  }, []);
+
+  const clearSelection = useCallback(() => {
+    setSelectedFeatureIds(new Set());
+  }, []);
+
+  const isFeatureSelected = useCallback(
+    (featureId: string) => selectedFeatureIds.has(featureId),
+    [selectedFeatureIds]
+  );
+
+  // Handle Escape key to exit selection mode
+  useEffect(() => {
+    const handleKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Escape' && isSelectionMode) {
+        exitSelectionMode();
+      }
+    };
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [isSelectionMode, exitSelectionMode]);
+
+  return {
+    isSelectionMode,
+    selectedFeatureIds,
+    selectedCount: selectedFeatureIds.size,
+    toggleSelectionMode,
+    toggleFeatureSelection,
+    selectAll,
+    clearSelection,
+    isFeatureSelected,
+    exitSelectionMode,
+  };
+}