Patch <title> in index.html with sitename during build

mdcms build now writes the sitename from config.yml into the <title> tag
of index.html. WhatsApp, Slack, and other link-preview crawlers read the
static HTML without executing JavaScript, so the title must be correct in
the raw file. Previously it was blank (or "MD-CMS" in older templates).

docs/unreleased.md

@@ -0,0 +1,10 @@
+# Unreleased changes
+
+Changes on `development` not yet merged to `main`.
+
+## Fixes
+
+- **`mdcms build` now patches `<title>` in `index.html` with `sitename` from `config.yml`.**
+  Previously the HTML `<title>` tag was a hardcoded blank (or "MD-CMS" in older templates),
+  so link previews in apps like WhatsApp showed the wrong name. After each build the tag
+  reflects the configured site name, which crawlers and preview scrapers read directly.

mdcms.py

@@ -412,6 +412,19 @@ def validate_assets(site_path: Path, cfg: dict) -> list:
 
 # ─── Core build logic ─────────────────────────────────────────
 
+_TITLE_RE = re.compile(r"<title>[^<]*</title>")
+
+
+def _patch_html_title(site_path: Path, sitename: str) -> None:
+    index = site_path / "index.html"
+    if not index.exists():
+        return
+    html = index.read_text(encoding="utf-8")
+    new_html = _TITLE_RE.sub(f"<title>{sitename}</title>", html, count=1)
+    if new_html != html:
+        index.write_text(new_html, encoding="utf-8")
+
+
 def run_build(site_path: Path):
     """Scan pages/ and posts/, write nav.yml and search.json. Raises ClickException on failure."""
     if not site_path.is_dir():
@@ -491,6 +504,8 @@ def run_build(site_path: Path):
     )
     click.echo(f"  Wrote search.json ({len(live_pages) + len(post_records)} entries)")
 
+    _patch_html_title(site_path, cfg.get("sitename", ""))
+
     pwa_enabled = str(cfg.get("pwa", "no")).lower() in ("yes", "true")
     if pwa_enabled:
         generate_pwa(site_path, cfg)