feat: Firebase integration, 3-step new trip form, full country list, and UI polish

- Connect Firestore for journal entries and visited countries (real-time onSnapshot)
- Connect Firebase Storage for photo uploads
- Add NewEntryForm: 3-step flow (trip details → photos → reflection questions)
- Expand country list to full world-atlas dataset (~240 countries) matching the map
- Filter city suggestions by selected country in both NewEntryForm and EditForm
- Redesign StatsPanel as floating horizontal card with donut chart and progress bar
- Center timeline layout with responsive side margins
- Replace "entry" language with "trip" throughout (Add trip, Save trip, Delete trip)
- Remove footer from Layout

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

src/lib/layout/selection.svelte.js

@@ -1,5 +1,5 @@
 import { db } from '../firebase.js';
-import { doc, onSnapshot, updateDoc, arrayUnion, arrayRemove } from 'firebase/firestore';
+import { doc, onSnapshot, setDoc, updateDoc, arrayUnion, arrayRemove } from 'firebase/firestore';
 
 let selected = $state(new Set());
 let totalCountries = $state(0);
@@ -20,29 +20,39 @@ export function initSelectionListener(uid) {
   });
 }
 
+const visitedRef = doc(db, 'visited', 'countries');
+
+onSnapshot(visitedRef, (snap) => {
+  if (snap.exists()) {
+    selected = new Set(snap.data().ids ?? []);
+  }
+});
+
+function persist() {
+  setDoc(visitedRef, { ids: [...selected] });
+}
+
 export function toggle(id) {
-  if (!_uid) return;
   const was = selected.has(id);
   const next = new Set(selected);
-  if (was) {
-    next.delete(id);
-  } else {
-    next.add(id);
-  }
+  if (was) next.delete(id);
+  else next.add(id);
   selected = next;
-  const userRef = doc(db, 'users', _uid);
-  if (was) {
-    updateDoc(userRef, { visitedCountries: arrayRemove(id) });
-  } else {
-    updateDoc(userRef, { visitedCountries: arrayUnion(id) });
+  persist();
+  if (_uid) {
+    const userRef = doc(db, 'users', _uid);
+    if (was) updateDoc(userRef, { visitedCountries: arrayRemove(id) });
+    else updateDoc(userRef, { visitedCountries: arrayUnion(id) });
   }
 }
 
 export function clearAll() {
-  if (!_uid) return;
   selected = new Set();
-  const userRef = doc(db, 'users', _uid);
-  updateDoc(userRef, { visitedCountries: [] });
+  persist();
+  if (_uid) {
+    const userRef = doc(db, 'users', _uid);
+    updateDoc(userRef, { visitedCountries: [] });
+  }
 }
 
 export function getSelected() {