UI refinement: toolbar icons, gradient CTA, Fraunces title, viewer polish

MarkdownToolbar:
- Replace text labels with SVG icons (Material Design paths)
- Group buttons with visual separators: text / headings / lists / code / links
- Add Strikethrough and Blockquote buttons (previously missing)
- Active state: tinted bg + ring instead of solid fill; hover lift shadow
- Toolbar container: pill shape with border (matches rest of UI)

editor-shared.css:
- btn-save: gradient + glow (linear-gradient(135deg, #6366f1, #4f46e5))
- title-input: borderless with Fraunces font and focus underline

NoteEditorView:
- Back button label standardized to "← Notes" (was "Back")
- Write/Preview tabs and toolbar now share the same row (flex-direction row)

NoteViewerView:
- Shimmer skeleton loader replaces plain "Loading..." text
- Note title uses Fraunces font at 2rem
- Edit button becomes primary gradient CTA
- Meta line shows clock/pencil SVG icons alongside timestamps
- Backlinks section: card grid with colored type badges (note=indigo, task=amber)

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

frontend/src/assets/editor-shared.css

@@ -51,15 +51,23 @@
   color: var(--color-primary);
 }
 .btn-save {
-  padding: 0.45rem 1rem;
-  background: var(--color-primary);
+  padding: 0.45rem 1.1rem;
+  background: linear-gradient(135deg, #6366f1, #4f46e5);
   color: #fff;
   border: none;
   border-radius: var(--radius-sm);
   cursor: pointer;
+  font-weight: 600;
+  font-size: 0.875rem;
+  box-shadow: 0 2px 8px rgba(99, 102, 241, 0.28);
+  transition: box-shadow 0.15s, opacity 0.15s;
+}
+.btn-save:hover:not(:disabled) {
+  box-shadow: 0 4px 14px rgba(99, 102, 241, 0.42);
+  opacity: 0.95;
 }
 .btn-save:disabled {
-  opacity: 0.6;
+  opacity: 0.55;
   cursor: default;
 }
 .btn-delete {
@@ -86,13 +94,26 @@
   color: var(--color-primary);
 }
 .title-input {
-  padding: 0.5rem 0.75rem;
-  border: 1px solid var(--color-input-border);
-  border-radius: var(--radius-sm);
-  font-size: 1.25rem;
-  font-weight: 600;
-  background: var(--color-bg-card);
+  padding: 0.4rem 0;
+  border: none;
+  border-bottom: 1.5px solid var(--color-border);
+  border-radius: 0;
+  font-size: 1.5rem;
+  font-weight: 700;
+  font-family: "Fraunces", Georgia, serif;
+  background: transparent;
   color: var(--color-text);
+  width: 100%;
+  transition: border-color 0.15s;
+}
+.title-input:focus {
+  outline: none;
+  border-bottom-color: var(--color-primary);
+}
+.title-input::placeholder {
+  color: var(--color-text-muted);
+  font-style: italic;
+  font-weight: 400;
 }
 .editor-tabs {
   display: flex;