fix(markdown): convert single newlines to hard breaks matching Obsidian behavior

Add _normalize_line_breaks() pre-processor that converts single \n to
two-spaces+\n (hard break) before mistune rendering, while protecting
fenced code blocks. This matches Obsidian's default 'Strict Line Breaks
= off' behavior where standalone lines render on separate rows instead
of merging into a single paragraph.

backend/main.py

@@ -908,6 +908,27 @@ def _convert_wikilinks(content: str, current_vault: str) -> str:
     return re.sub(pattern, _replace, content)
 
 
+def _normalize_line_breaks(text: str) -> str:
+    """Convert single newlines to hard breaks (matching Obsidian default behavior).
+
+    In standard Markdown, a single ``\\n`` is a "soft break" — it renders as a space,
+    not a visible line break.  Obsidian defaults to treating single newlines as hard
+    breaks (equivalent to ``<br>``).  This function pre-processes the Markdown source
+    so that mistune renders standalone lines on separate rows, while still honouring
+    blank lines as paragraph separators.
+
+    Fenced code blocks (`` ``` ``) are left untouched so their internal newlines are
+    preserved verbatim.
+    """
+    parts = re.split(r"(```[\s\S]*?```)", text)
+    for i, part in enumerate(parts):
+        if part.startswith("```"):
+            continue  # Protect fenced code blocks
+        # Single \n (not preceded or followed by another \n) → two spaces + \n
+        parts[i] = re.sub(r"(?<!\n)\n(?!\n)", "  \n", part)
+    return "".join(parts)
+
+
 def _render_markdown(raw_md: str, vault_name: str, current_file_path: Path | None = None) -> str:
     """Render a markdown string to HTML with wikilink and image support.
 
@@ -936,6 +957,9 @@ def _render_markdown(raw_md: str, vault_name: str, current_file_path: Path | Non
     # Convert wikilinks
     converted = _convert_wikilinks(raw_md, vault_name)
     
+    # Normalize line breaks to match Obsidian behavior (single \n → hard break)
+    converted = _normalize_line_breaks(converted)
+    
     rendered = _markdown_renderer(converted)
     
     # Add heading IDs for TOC navigation