feat(web): AVM v2 upgraded valuation dashboard (TEC-2763)

R5.4 ships the upgraded AVM UI behind the `avm_v2` A/B flag. When the
flag is on, the dashboard exposes:

- Tab switch between single valuation and multi-property compare
- Waterfall drivers chart (ValueDriversChart) alongside the existing
  horizontal bar breakdown
- Mapbox comparables map with similarity-coloured markers and an
  optional highlighted subject pin
- Confidence interval + range bar and PDF export remain available
- Valuation history chart surface unchanged (still lazy-loaded)

Flag plumbing (useAvmV2Flag):
- NEXT_PUBLIC_FEATURE_AVM_V2=1 enables by default
- `?avm_v2=1|0` URL param forces + persists to localStorage
- safe localStorage handling (no throw when storage is blocked)

Tests: comparables-map, value-drivers-chart, use-avm-v2-flag specs
added. Pre-existing "Yếu tố chính" assertion in valuation-results.spec
updated to match the current copy ("Yếu tố ảnh hưởng giá") so the
valuation suite is green (7 files, 52 tests).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/lib/hooks/use-avm-v2-flag.ts

@@ -0,0 +1,55 @@
+'use client';
+
+import { useEffect, useState } from 'react';
+
+const LOCAL_STORAGE_KEY = 'goodgo:avm_v2';
+const QUERY_PARAM = 'avm_v2';
+
+function readEnvDefault(): boolean {
+  const raw = process.env['NEXT_PUBLIC_FEATURE_AVM_V2'];
+  if (!raw) return false;
+  return raw === '1' || raw.toLowerCase() === 'true';
+}
+
+function readOverride(): boolean | null {
+  if (typeof window === 'undefined') return null;
+
+  const params = new URLSearchParams(window.location.search);
+  const qp = params.get(QUERY_PARAM);
+  if (qp === '1' || qp === 'true') {
+    try {
+      window.localStorage.setItem(LOCAL_STORAGE_KEY, '1');
+    } catch {
+      // localStorage may be blocked — ignore
+    }
+    return true;
+  }
+  if (qp === '0' || qp === 'false') {
+    try {
+      window.localStorage.setItem(LOCAL_STORAGE_KEY, '0');
+    } catch {
+      // localStorage may be blocked — ignore
+    }
+    return false;
+  }
+
+  try {
+    const stored = window.localStorage.getItem(LOCAL_STORAGE_KEY);
+    if (stored === '1') return true;
+    if (stored === '0') return false;
+  } catch {
+    // ignore
+  }
+  return null;
+}
+
+export function useAvmV2Flag(): boolean {
+  const [enabled, setEnabled] = useState<boolean>(readEnvDefault());
+
+  useEffect(() => {
+    const override = readOverride();
+    setEnabled(override ?? readEnvDefault());
+  }, []);
+
+  return enabled;
+}