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>

config/default.py

@@ -8,3 +8,7 @@ CACHE_DIR = os.path.expanduser('~/lib/data/tfl/cache')
 
 # TransXChange timetable file for the Circle Line
 CIRCLE_LINE_XML = os.path.join(TFL_DATA_DIR, 'output_txc_01CIR_.xml')
+
+# Default connection window (minutes) between Paddington arrival and St Pancras departure
+DEFAULT_MIN_CONNECTION = 70
+DEFAULT_MAX_CONNECTION = 150