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/base.html

@@ -192,7 +192,6 @@
 
     /* Card helpers */
     .card > h2:first-child { margin-top: 0; }
-    .card-scroll { overflow-x: auto; }
 
     /* Form groups */
     .form-group    { margin-bottom: 1.2rem; }