Make Back and Edit navigation consistently styled as bordered buttons

Previously .btn-back and .btn-edit in editor/viewer toolbars were plain
primary-colored links while all other toolbar actions were proper buttons.
All four views now use the same bordered button style: subtle border,
secondary text color, hover highlights primary border/text — matching
the existing convert/assist-toggle button aesthetic.

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

frontend/src/views/NoteEditorView.vue

@@ -476,7 +476,19 @@ onUnmounted(() => window.removeEventListener("beforeunload", onBeforeUnload));
   align-items: center;
 }
 .btn-back {
+  display: inline-flex;
+  align-items: center;
+  padding: 0.45rem 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  background: none;
+  color: var(--color-text-secondary);
   text-decoration: none;
+  cursor: pointer;
+  font-size: 0.9rem;
+}
+.btn-back:hover {
+  border-color: var(--color-primary);
   color: var(--color-primary);
 }
 .btn-save {

frontend/src/views/NoteViewerView.vue

@@ -197,7 +197,20 @@ async function convertToTask() {
 }
 .btn-back,
 .btn-edit {
+  display: inline-flex;
+  align-items: center;
+  padding: 0.45rem 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  background: none;
+  color: var(--color-text-secondary);
   text-decoration: none;
+  cursor: pointer;
+  font-size: 0.9rem;
+}
+.btn-back:hover,
+.btn-edit:hover {
+  border-color: var(--color-primary);
   color: var(--color-primary);
 }
 .btn-convert {

frontend/src/views/TaskEditorView.vue

@@ -525,7 +525,19 @@ onUnmounted(() => window.removeEventListener("beforeunload", onBeforeUnload));
   align-items: center;
 }
 .btn-back {
+  display: inline-flex;
+  align-items: center;
+  padding: 0.45rem 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  background: none;
+  color: var(--color-text-secondary);
   text-decoration: none;
+  cursor: pointer;
+  font-size: 0.9rem;
+}
+.btn-back:hover {
+  border-color: var(--color-primary);
   color: var(--color-primary);
 }
 .btn-save {

frontend/src/views/TaskViewerView.vue

@@ -227,7 +227,20 @@ function onTagClick(tag: string) {
 }
 .btn-back,
 .btn-edit {
+  display: inline-flex;
+  align-items: center;
+  padding: 0.45rem 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  background: none;
+  color: var(--color-text-secondary);
   text-decoration: none;
+  cursor: pointer;
+  font-size: 0.9rem;
+}
+.btn-back:hover,
+.btn-edit:hover {
+  border-color: var(--color-primary);
   color: var(--color-primary);
 }
 .btn-convert {