feat: implement package 1 session and validation feedback

- add session validation_errors bootstrap initialization

- render global flash + validation messages in header

- remove footer alert-based flash handling

- persist structured validation errors across save handlers

- mark NEXT_STEPS package 1 tasks as done

app/assets/css/app.css

@@ -77,6 +77,42 @@ main {
     min-height: calc(100vh - 200px);
 }
 
+.flash-stack {
+    display: grid;
+    gap: 10px;
+    margin: 0 auto 18px;
+    max-width: 1200px;
+}
+
+.flash-message {
+    border-radius: 8px;
+    border: 1px solid transparent;
+    padding: 12px 14px;
+    background: #f8fafc;
+}
+
+.flash-message--success {
+    border-color: #99dfba;
+    background: #ebf9f1;
+    color: #165938;
+}
+
+.flash-message--error {
+    border-color: #efb4b4;
+    background: #fff1f1;
+    color: #8a1f1f;
+}
+
+.flash-message__text {
+    margin: 0;
+    font-weight: 600;
+}
+
+.flash-message__list {
+    margin: 8px 0 0 18px;
+    padding: 0;
+}
+
 /* Shared components -------------------------------------------------- */
 .filter-form {
     margin: 20px 0;
@@ -192,4 +228,4 @@ main {
 
 footer>p {
     margin-bottom: 0;
-}
+}