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/views/NoteEditorView.vue

@@ -321,7 +321,7 @@ onUnmounted(() => assist.clearSelection());
   <main class="editor-page note-editor-page">
     <div class="editor-header">
       <div class="toolbar">
-        <router-link to="/notes" class="btn-back">Back</router-link>
+        <router-link to="/notes" class="btn-back">← Notes</router-link>
         <button class="btn-save" @click="save" :disabled="saving">
           {{ saving ? "Saving..." : "Save" }}
         </button>
@@ -575,8 +575,12 @@ onUnmounted(() => assist.clearSelection());
 
 .body-tabs-row {
   display: flex;
-  flex-direction: column;
-  gap: 0.25rem;
+  flex-direction: row;
+  align-items: center;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+  padding-bottom: 0.5rem;
+  border-bottom: 1px solid var(--color-border);
 }
 
 .editor-tabs {
@@ -586,7 +590,7 @@ onUnmounted(() => assist.clearSelection());
   border-radius: 8px;
   padding: 2px;
   gap: 2px;
-  align-self: flex-start;
+  flex-shrink: 0;
 }
 
 .tab {