feat: add styled 404/500 error pages with i18n support

Custom error templates extending base.html with centered layout.
404 is context-aware: detects /markets/{country}/{city} paths and
shows city-specific message with link back to country overview.
Both pages support EN/DE translations.

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

web/src/padelnomics/app.py

@@ -5,7 +5,7 @@ import json
 import time
 from pathlib import Path
 
-from quart import Quart, Response, abort, g, redirect, request, session, url_for
+from quart import Quart, Response, abort, g, redirect, render_template, request, session, url_for
 
 from .analytics import close_analytics_db, open_analytics_db
 from .core import (
@@ -270,12 +270,39 @@ def create_app() -> Quart:
         from .sitemap import sitemap_response
         return await sitemap_response(config.BASE_URL)
 
-    # Ensure unhandled exceptions are always logged (Granian doesn't show
-    # Quart's debug error page, so without this 500s are silent).
+    # -------------------------------------------------------------------------
+    # Error pages
+    # -------------------------------------------------------------------------
+
+    def _error_lang() -> str:
+        """Best-effort language from URL path prefix (no g.lang in error handlers)."""
+        path = request.path
+        if path.startswith("/de/"):
+            return "de"
+        return "en"
+
+    @app.errorhandler(404)
+    async def handle_404(error):
+        import re
+        lang = _error_lang()
+        t = get_translations(lang)
+        country_slug = None
+        country_name = None
+        m = re.match(r"^/(?:en|de)/markets/([^/]+)/[^/]+/?$", request.path)
+        if m:
+            country_slug = m.group(1)
+            country_name = country_slug.replace("-", " ").title()
+        return await render_template(
+            "404.html", lang=lang, t=t, country_slug=country_slug,
+            country_name=country_name or "",
+        ), 404
+
     @app.errorhandler(500)
     async def handle_500(error):
         app.logger.exception("Unhandled 500 error: %s", error)
-        return "Internal Server Error", 500
+        lang = _error_lang()
+        t = get_translations(lang)
+        return await render_template("500.html", lang=lang, t=t), 500
 
     # Health check
     @app.route("/health")

web/src/padelnomics/locales/de.json

@@ -1825,5 +1825,16 @@
   "affiliate_pros_label": "Vorteile",
   "affiliate_cons_label": "Nachteile",
   "affiliate_at_retailer": "bei {retailer}",
-  "affiliate_our_picks": "Unsere Empfehlungen"
+  "affiliate_our_picks": "Unsere Empfehlungen",
+
+  "error_404_title": "Seite nicht gefunden",
+  "error_404_heading": "Diese Seite gibt es nicht",
+  "error_404_message": "Die gesuchte Seite wurde verschoben oder existiert noch nicht.",
+  "error_404_city_message": "Die Marktanalyse für diese Stadt ist noch nicht verfügbar.",
+  "error_404_back_home": "Zur Startseite",
+  "error_404_back_country": "Zurück zur {country}-Übersicht",
+  "error_500_title": "Etwas ist schiefgelaufen",
+  "error_500_heading": "Etwas ist schiefgelaufen",
+  "error_500_message": "Wir arbeiten an einer Lösung. Bitte versuche es gleich noch einmal.",
+  "error_500_back_home": "Zur Startseite"
 }

web/src/padelnomics/locales/en.json

@@ -1828,5 +1828,16 @@
   "affiliate_pros_label": "Pros",
   "affiliate_cons_label": "Cons",
   "affiliate_at_retailer": "at {retailer}",
-  "affiliate_our_picks": "Our picks"
+  "affiliate_our_picks": "Our picks",
+
+  "error_404_title": "Page Not Found",
+  "error_404_heading": "This page doesn't exist",
+  "error_404_message": "The page you're looking for may have been moved or doesn't exist yet.",
+  "error_404_city_message": "The market analysis for this city isn't available yet.",
+  "error_404_back_home": "Back to Home",
+  "error_404_back_country": "Back to {country} overview",
+  "error_500_title": "Something Went Wrong",
+  "error_500_heading": "Something went wrong",
+  "error_500_message": "We're working on fixing this. Please try again in a moment.",
+  "error_500_back_home": "Back to Home"
 }

web/src/padelnomics/templates/404.html

@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+
+{% block title %}{{ t.error_404_title }} — {{ config.APP_NAME }}{% endblock %}
+
+{% block content %}
+<div class="container-page py-12">
+  <div style="max-width:28rem;margin:0 auto;text-align:center;">
+    <p style="font-size:6rem;font-weight:800;line-height:1;color:var(--slate);opacity:0.3;margin:0;">404</p>
+    <h1 class="text-navy" style="font-size:1.5rem;font-weight:700;margin:1rem 0 0.5rem;">{{ t.error_404_heading }}</h1>
+    {% if country_slug %}
+      <p class="text-slate" style="font-size:1rem;line-height:1.6;">{{ t.error_404_city_message }}</p>
+      <a href="/{{ lang }}/markets/{{ country_slug }}" class="btn" style="margin-top:1.5rem;display:inline-block;">
+        {{ t.error_404_back_country.replace('{country}', country_name) }}
+      </a>
+    {% else %}
+      <p class="text-slate" style="font-size:1rem;line-height:1.6;">{{ t.error_404_message }}</p>
+      <a href="/{{ lang }}" class="btn" style="margin-top:1.5rem;display:inline-block;">
+        {{ t.error_404_back_home }}
+      </a>
+    {% endif %}
+  </div>
+</div>
+{% endblock %}

web/src/padelnomics/templates/500.html

@@ -0,0 +1,16 @@
+{% extends "base.html" %}
+
+{% block title %}{{ t.error_500_title }} — {{ config.APP_NAME }}{% endblock %}
+
+{% block content %}
+<div class="container-page py-12">
+  <div style="max-width:28rem;margin:0 auto;text-align:center;">
+    <p style="font-size:6rem;font-weight:800;line-height:1;color:var(--slate);opacity:0.3;margin:0;">500</p>
+    <h1 class="text-navy" style="font-size:1.5rem;font-weight:700;margin:1rem 0 0.5rem;">{{ t.error_500_heading }}</h1>
+    <p class="text-slate" style="font-size:1rem;line-height:1.6;">{{ t.error_500_message }}</p>
+    <a href="/{{ lang }}" class="btn" style="margin-top:1.5rem;display:inline-block;">
+      {{ t.error_500_back_home }}
+    </a>
+  </div>
+</div>
+{% endblock %}