Add web frontend and refactor core to use OsmError

- Refactor core.py: replace sys.exit() calls with OsmError exceptions
  so the library is safe to use from Flask and other callers
- Add fetch_sibling_routes and fetch_route_master_routes to core.py
- Add Flask web frontend (web/app.py, templates, static assets):
  - Map view with Leaflet; full route drawn in grey on load
  - Sidebar stop list; active-slot UX for From/To selection
  - Segment preview and download (full route or selected segment)
  - Include-stops toggle applied client-side
  - Bookmarkable URLs: GET /<relation_id>
  - Clear selection button
  - Other directions panel (sibling routes from same route_master)
  - route_master handling: draws all member routes in colour on map
    with links to each individual direction
- Add SVG favicon
- Add py.typed marker; add .gitignore

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

tests/test_osm_pt_geojson.py

@@ -8,6 +8,7 @@ from click.testing import CliRunner
 
 from osm_geojson.pt import core
 from osm_geojson.pt.cli import cli, output_geojson
+from osm_geojson.pt.core import OsmError
 
 FIXTURES = Path(__file__).parent / "fixtures"
 FULL_URL = "https://www.openstreetmap.org/api/0.6/relation/15083963/full.json"
@@ -57,8 +58,8 @@ def test_parse_elements_tags(parsed: tuple) -> None:
 
 
 def test_parse_elements_unknown_relation(full_data: dict) -> None:
-    """Requesting a relation ID not present in the response exits with an error."""
-    with pytest.raises(SystemExit):
+    """Requesting a relation ID not present in the response raises OsmError."""
+    with pytest.raises(OsmError):
         core.parse_elements(full_data, 9999999)