feat: remove workspace system, add pinned flows and label renames

Replace workspace system with pinned flows API (pin/unpin/list/reorder).
Rename user-facing labels: Tree→Flow, Procedure→Project. Add sidebar
nav sub-items for flow type filtering. Remove 11 workspace files,
add migrations 037-038, clean all workspace references.

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

backend/alembic/versions/037_add_user_pinned_trees.py

@@ -0,0 +1,40 @@
+"""Add user_pinned_trees table
+
+Revision ID: 037
+Revises: 036
+Create Date: 2026-02-15
+
+Adds pinned flows feature for sidebar favorites.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID
+
+
+# revision identifiers, used by Alembic.
+revision: str = '037'
+down_revision: Union[str, None] = '036'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'user_pinned_trees',
+        sa.Column('id', UUID(as_uuid=True), primary_key=True, server_default=sa.text('gen_random_uuid()')),
+        sa.Column('user_id', UUID(as_uuid=True), sa.ForeignKey('users.id', ondelete='CASCADE'), nullable=False),
+        sa.Column('tree_id', UUID(as_uuid=True), sa.ForeignKey('trees.id', ondelete='CASCADE'), nullable=False),
+        sa.Column('display_order', sa.Integer(), nullable=False, server_default='0'),
+        sa.Column('pinned_at', sa.DateTime(timezone=True), server_default=sa.text('NOW()')),
+        sa.UniqueConstraint('user_id', 'tree_id', name='uq_user_pinned_tree'),
+    )
+    op.create_index('idx_user_pinned_trees_user', 'user_pinned_trees', ['user_id'])
+    op.create_index('idx_user_pinned_trees_tree', 'user_pinned_trees', ['tree_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('idx_user_pinned_trees_tree', 'user_pinned_trees')
+    op.drop_index('idx_user_pinned_trees_user', 'user_pinned_trees')
+    op.drop_table('user_pinned_trees')