fix(render): use forward slashes in save-path footer for Windows

The footer line `📎 Raw results saved to ~/Documents\Last30Days\…`
mangled the home-relative path on Windows because `f"~/{relative}"`
stringifies a `pathlib.Path` with the OS-native separator. The result
mixes a Unix tilde with backslashes, which neither File Explorer,
PowerShell, nor a `file://` URI can resolve.

`Path.as_posix()` always returns forward slashes, which is the
convention `~/`-prefixed paths require on every platform. macOS and
Linux output is unchanged because their separator is already `/`.

Repro on Windows:
  python3 last30days.py "anything" --emit=compact --save-dir="$HOME/Documents/Last30Days"
  # before: 📎 Raw results saved to ~/Documents\Last30Days\anything-raw.md
  # after:  📎 Raw results saved to ~/Documents/Last30Days/anything-raw.md

skills/last30days/scripts/last30days.py

@@ -195,7 +195,7 @@ def compute_save_path_display(save_dir: str, topic: str, suffix: str, emit: str)
     try:
         home = _Path.home().resolve()
         relative = raw.relative_to(home)
-        return f"~/{relative}"
+        return f"~/{relative.as_posix()}"
     except ValueError:
         return str(raw)