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>

web/static/style.css

@@ -0,0 +1,68 @@
+html, body {
+  height: 100%;
+  overflow: hidden;
+}
+
+#main-row {
+  height: calc(100vh - 56px); /* subtract navbar height */
+}
+
+#sidebar {
+  height: 100%;
+  overflow-y: auto;
+  padding: 1rem;
+}
+
+#map {
+  height: 100%;
+}
+
+.slot-box {
+  border: 2px solid #dee2e6;
+  border-radius: 6px;
+  padding: 0.4rem 0.6rem;
+  cursor: pointer;
+  min-height: 2.2rem;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  transition: border-color 0.15s;
+}
+
+.slot-box.active {
+  border-color: #0d6efd;
+  background: #f0f5ff;
+}
+
+.slot-box .placeholder {
+  color: #adb5bd;
+  font-style: italic;
+}
+
+.slot-box .pencil {
+  color: #0d6efd;
+  font-size: 0.8rem;
+}
+
+#stop-list .stop-item {
+  cursor: pointer;
+  padding: 0.25rem 0.5rem;
+  border-radius: 4px;
+  font-size: 0.9rem;
+}
+
+#stop-list .stop-item:hover {
+  background: #e9ecef;
+}
+
+#stop-list .stop-item.is-from {
+  background: #d1e7dd;
+}
+
+#stop-list .stop-item.is-to {
+  background: #f8d7da;
+}
+
+.leaflet-container {
+  font-family: inherit;
+}