Tidy results table layout and centralise connection defaults

- Redesign results table from 8 columns to 4 (National Rail, Transfer,
  Eurostar, Total), making GWR and Eurostar legs consistent with each other
- Move CET label next to Paris arrival time; show duration · train number
  on one line below
- Move "Too early" label into the National Rail column for unreachable rows
- Remove horizontal scrollbar (drop card-scroll / overflow-x: auto)
- Add DEFAULT_MIN_CONNECTION / DEFAULT_MAX_CONNECTION to config/default.py
  (70 / 150 min); remove all hardcoded fallback values from app.py and
  templates
- Redirect to clean URL when both connection params equal their defaults;
  omit params from all generated links when at default values

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

templates/index.html

@@ -49,7 +49,7 @@
       </label>
       <select id="min_connection" name="min_connection" class="form-control">
         {% for mins in valid_min_connections %}
-          <option value="{{ mins }}" {% if mins == 70 %}selected{% endif %}>{{ mins }} min</option>
+          <option value="{{ mins }}" {% if mins == default_min_connection %}selected{% endif %}>{{ mins }} min</option>
         {% endfor %}
       </select>
     </div>
@@ -60,7 +60,7 @@
       </label>
       <select id="max_connection" name="max_connection" class="form-control">
         {% for mins in valid_max_connections %}
-          <option value="{{ mins }}" {% if mins == 150 %}selected{% endif %}>{{ mins }} min</option>
+          <option value="{{ mins }}" {% if mins == default_max_connection %}selected{% endif %}>{{ mins }} min</option>
         {% endfor %}
       </select>
     </div>