refactor(frontend): CSS pay-down, derive batch (milestone 302 step 4) — page-header, error-msg/state-msg/empty-msg, empty-title/empty-sub, required, field-hint recipes into components.css; form-buttons into rules-shared.css; scoped copies trimmed to remainders/overrides; the three views' .input becomes fs-input + width/box-sizing remainder (canon #2336)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/views/SnippetListView.vue

@@ -520,13 +520,7 @@ function usageTitle(s: SnippetListItem): string {
 <style src="@/assets/dup-report.css" />
 <style scoped>
 .page-header {
-  display: flex;
-  justify-content: space-between;
-  align-items: center;
-  margin-bottom: 0.35rem;
-}
-.page-header h1 {
-  margin: 0;
+  margin-bottom: 0.35rem;   /* tighter than the shared recipe: .page-sub follows */
 }
 .page-sub {
   margin: 0 0 1.25rem;
@@ -616,8 +610,6 @@ function usageTitle(s: SnippetListItem): string {
 }
 
 .error-msg {
-  color: var(--fs-error);
-  font-size: 0.9rem;
   margin-top: 1rem;
 }
 
@@ -632,15 +624,7 @@ function usageTitle(s: SnippetListItem): string {
   margin-bottom: 0.75rem;
   opacity: 0.35;
 }
-.empty-title {
-  font-size: 1rem;
-  font-weight: 500;
-  color: var(--fs-text-secondary);
-  margin: 0 0 0.35rem;
-}
 .empty-sub {
-  font-size: 0.85rem;
-  margin: 0 0 1rem;
   max-width: 44ch;
   margin-inline: auto;
   line-height: 1.5;