Add notebook delete, fix button styling, rename Share button

- Add delete notebook handler with ownership check and CASCADE delete
- Rename "Create Share Link" to "Share"
- Fix action button heights: use inline-flex + align-items for
  consistent sizing across <a> and <button> elements

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/notebook.html

@@ -6,7 +6,11 @@
     <a href="{{.PDFLink}}" class="btn">Download PDF</a>
     {{if not .Shared}}
     <form method="POST" action="/notebooks/{{.ID}}/share" style="display:inline;">
-        <button type="submit" class="btn">Create Share Link</button>
+        <button type="submit" class="btn">Share</button>
+    </form>
+    <form method="POST" action="/notebooks/{{.ID}}/delete" style="display:inline;"
+          onsubmit="return confirm('Delete this notebook from the server?');">
+        <button type="submit" class="btn" style="color: #c62828; border-color: #c62828;">Delete</button>
     </form>
     {{end}}
 </div>