fix(settings): fail closed on blank loan input

tests/test_settings_validation_playwright.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from playwright.sync_api import expect, sync_playwright
+
+BASE_URL = "http://127.0.0.1:8000"
+
+
+def test_settings_reject_invalid_loan_amount_without_silent_zero_fallback() -> None:
+    with sync_playwright() as p:
+        browser = p.chromium.launch(headless=True)
+        page = browser.new_page(viewport={"width": 1440, "height": 1000})
+
+        page.goto(BASE_URL, wait_until="domcontentloaded", timeout=30000)
+        expect(page.locator("text=Create a private workspace URL").first).to_be_visible(timeout=10000)
+        page.get_by_role("button", name="Get started").click()
+        page.wait_for_url(f"{BASE_URL}/*", timeout=15000)
+        workspace_url = page.url
+
+        page.goto(f"{workspace_url}/settings", wait_until="domcontentloaded", timeout=30000)
+        expect(page.locator("text=Settings").first).to_be_visible(timeout=15000)
+        loan_input = page.get_by_label("Loan amount ($)")
+        loan_input.fill("")
+        loan_input.press("Tab")
+
+        expect(page.locator("text=INVALID").first).to_be_visible(timeout=15000)
+        expect(page.locator("text=Loan amount must be zero or greater").first).to_be_visible(timeout=15000)
+
+        page.get_by_role("button", name="Save settings").click()
+        expect(page.locator("text=Validation error: Loan amount must be zero or greater").first).to_be_visible(
+            timeout=15000
+        )
+
+        settings_text = page.locator("body").inner_text(timeout=15000)
+        assert "LTV=0.0%" not in settings_text
+        assert "RuntimeError" not in settings_text
+        assert "Server error" not in settings_text
+        assert "Traceback" not in settings_text
+
+        browser.close()