feat: implement timer screens and navigation

client/src/views/Timer/Timer.css

@@ -10,8 +10,29 @@
   font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto,
     'Helvetica Neue', Arial, sans-serif;
   display: flex;
-  align-items: center;
-  justify-content: center;
+  flex-direction: column;
+}
+
+.page-header {
+  background-color: #ffffff;
+  padding: 16px;
+  border-bottom: 1px solid #e0e0e0;
+  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.04);
+  width: 100%;
+  min-height: 72px;
+  position: relative;
+}
+
+.page-header h1 {
+  font-size: 32px;
+  font-weight: 400;
+  color: #212121;
+  letter-spacing: -0.5px;
+  margin: 0;
+  position: absolute;
+  left: 16px;
+  top: 50%;
+  transform: translateY(-50%);
 }
 
 .timer-screen {
@@ -21,6 +42,47 @@
   display: flex;
   flex-direction: column;
   align-items: center;
+  margin: 0 auto;
+  flex: 1;
+  justify-content: center;
+}
+
+.confirm-overlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(15, 23, 42, 0.35);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 16px;
+  z-index: 1000;
+}
+
+.confirm-card {
+  width: 100%;
+  max-width: 380px;
+  background: #ffffff;
+  border-radius: 16px;
+  padding: 18px;
+  box-shadow: 0 12px 28px rgba(0, 0, 0, 0.18);
+}
+
+.confirm-card h3 {
+  font-size: 18px;
+  color: #212121;
+  margin-bottom: 8px;
+}
+
+.confirm-card p {
+  font-size: 14px;
+  color: #666;
+  margin-bottom: 16px;
+}
+
+.confirm-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
 }
 
 .timer-display-card {
@@ -87,6 +149,31 @@
   height: 48px;
 }
 
+.btn-back {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: 6px;
+  min-height: 40px;
+  min-width: 96px;
+  padding: 0 14px;
+  position: absolute;
+  right: 16px;
+  top: 50%;
+  transform: translateY(-50%);
+  border-radius: 999px;
+  border: 2px solid #f44336;
+  background-color: transparent;
+  color: #f44336;
+  font-size: 13px;
+  font-weight: 500;
+  cursor: pointer;
+}
+
+.btn-back:hover {
+  background-color: rgba(244, 67, 54, 0.10);
+}
+
 .btn-timer {
   display: flex;
   align-items: center;