feat: add dual-mode tree editor with Code Mode, variables, and markdown sync

Implements the full dual-mode tree editor (Plan Phases 1-5):

Backend:
- JSONB↔Markdown bidirectional serializer/parser with mistune
- Markdown validator with line/column error reporting
- 3 API endpoints: export-markdown, import-markdown, validate-markdown
- Variable extraction/resolution service ([USER_INPUT], [VAR], [SAVE_AS])
- Session variables JSONB column (migration 028)
- 39 tree markdown tests + variable service tests (403 total passing)

Frontend:
- Monaco-based Code Mode with custom Monarch tokenizer and dark theme
- Autocomplete for @node_id refs, type values, variable names
- Debounced validation (800ms) with inline Monaco error markers
- Syntax help panel (absolute overlay, toggleable)
- Starter template for new trees with valid cross-references
- Bidirectional metadata sync (name/description/category/tags frontmatter)
- Synchronous tree→markdown serializer (fixes async race condition)
- Pre-save validation blocks save on broken refs or missing tree name
- Mode-aware undo/redo: Monaco native in Code Mode, throttled zundo in Flow Mode
- Variable prompt modal and frontend resolver for session navigation

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

backend/tests/test_variable_service.py

@@ -0,0 +1,119 @@
+"""Tests for variable extraction and resolution service."""
+import pytest
+
+from app.services.variable_service import extract_variables, resolve_variables
+
+
+TREE_WITH_VARIABLES = {
+    "id": "root",
+    "type": "decision",
+    "question": "What is the hostname of [USER_INPUT:hostname]?",
+    "help_text": "Enter the server hostname",
+    "options": [
+        {"id": "opt1", "label": "Check [VAR:hostname]", "next_node_id": "action1"},
+    ],
+    "children": [
+        {
+            "id": "action1",
+            "type": "action",
+            "title": "Ping [VAR:hostname]",
+            "description": "Run diagnostics on [VAR:hostname].\n\n[SAVE_AS:test_results]",
+            "commands": ["ping [VAR:hostname]"],
+            "expected_outcome": "Replies from [VAR:hostname]",
+            "next_node_id": "solution1",
+            "children": [
+                {
+                    "id": "solution1",
+                    "type": "solution",
+                    "title": "Resolved",
+                    "description": "Issue on [VAR:hostname] resolved.",
+                    "resolution_steps": [
+                        "Document results for [VAR:hostname]",
+                        "Save as [SAVE_AS:final_result]"
+                    ],
+                    "solution": "Resolved",
+                }
+            ],
+        }
+    ],
+}
+
+TREE_WITHOUT_VARIABLES = {
+    "id": "root",
+    "type": "solution",
+    "title": "Simple Fix",
+    "description": "Just restart it.",
+    "solution": "Restart",
+}
+
+
+class TestExtractVariables:
+    def test_extracts_user_input(self):
+        variables = extract_variables(TREE_WITH_VARIABLES)
+        user_inputs = [v for v in variables if v.kind == "user_input"]
+        assert len(user_inputs) == 1
+        assert user_inputs[0].name == "hostname"
+        assert user_inputs[0].node_id == "root"
+
+    def test_extracts_var_references(self):
+        variables = extract_variables(TREE_WITH_VARIABLES)
+        refs = [v for v in variables if v.kind == "reference"]
+        assert len(refs) >= 4  # hostname used in multiple places
+
+    def test_extracts_save_as(self):
+        variables = extract_variables(TREE_WITH_VARIABLES)
+        saves = [v for v in variables if v.kind == "save_as"]
+        assert len(saves) == 2
+        names = {v.name for v in saves}
+        assert "test_results" in names
+        assert "final_result" in names
+
+    def test_no_variables(self):
+        variables = extract_variables(TREE_WITHOUT_VARIABLES)
+        assert len(variables) == 0
+
+    def test_extracts_from_commands(self):
+        variables = extract_variables(TREE_WITH_VARIABLES)
+        cmd_vars = [v for v in variables if v.node_id == "action1" and v.kind == "reference"]
+        # hostname in title, description, commands, expected_outcome
+        assert len(cmd_vars) >= 3
+
+
+class TestResolveVariables:
+    def test_resolves_var_reference(self):
+        text = "Ping [VAR:hostname] now"
+        result = resolve_variables(text, {"hostname": "server01"})
+        assert result == "Ping server01 now"
+
+    def test_resolves_user_input(self):
+        text = "Server: [USER_INPUT:hostname]"
+        result = resolve_variables(text, {"hostname": "server01"})
+        assert result == "Server: server01"
+
+    def test_removes_save_as(self):
+        text = "Done [SAVE_AS:result] here"
+        result = resolve_variables(text, {})
+        assert result == "Done  here"
+
+    def test_unresolved_var_preserved(self):
+        text = "Server: [VAR:unknown_var]"
+        result = resolve_variables(text, {})
+        assert result == "Server: [VAR:unknown_var]"
+
+    def test_multiple_variables(self):
+        text = "[VAR:hostname] ([VAR:ip]) - [USER_INPUT:ticket]"
+        result = resolve_variables(text, {
+            "hostname": "server01",
+            "ip": "10.0.0.1",
+            "ticket": "INC001",
+        })
+        assert result == "server01 (10.0.0.1) - INC001"
+
+    def test_empty_variables_dict(self):
+        text = "No vars here"
+        result = resolve_variables(text, {})
+        assert result == "No vars here"
+
+    def test_empty_text(self):
+        result = resolve_variables("", {"hostname": "server01"})
+        assert result == ""