fix(ui): route every home lookup through the one that knows Windows

`path_display` opens by calling itself the one place a path is measured
against `~`, because three rows spelling that check their own way is what
#544 was. Three sites in src/ui had since gone back to reading the
environment directly, and two of them read only `HOME` — the variable
#544 recorded as "often unset" on Windows:

- the file tree's fallback root, so a Windows window with no resolved
  root drew an empty tree instead of the home directory;
- `expand_path`, so a `~/`-prefixed theme in config fell through to
  `themes_dir().join("~/theme.json")` — a path that cannot exist, and no
  error to say why.

The third (the SFTP download directory) already checked `USERPROFILE`,
but as a second copy of the canonical lookup under the same name, minus
its empty-string filter: `HOME=` set-but-empty resolved downloads to a
relative `Downloads`. It keeps its `.` last resort, which is a real
difference worth stating rather than deleting — a download has to be
offered somewhere, while every other caller wants the `None`.

No behaviour change on Unix, where `HOME` was already the answer.

src/ui/file_tree.rs

@@ -782,9 +782,9 @@ impl Tty7App {
         }
         if roots.is_empty()
             && id.is_local()
-            && let Some(home) = std::env::var_os("HOME")
+            && let Some(home) = crate::ui::path_display::local_home()
         {
-            roots.push(PathBuf::from(home));
+            roots.push(home);
         }
         let _ = window;
         let Some(code) = self.tab_code_mut_or_init() else {

src/ui/presets.rs

@@ -941,9 +941,9 @@ impl FillFile {
 fn expand_path(p: &str) -> PathBuf {
     let p = p.trim();
     if let Some(rest) = p.strip_prefix("~/")
-        && let Some(home) = std::env::var_os("HOME")
+        && let Some(home) = crate::ui::path_display::local_home()
     {
-        return PathBuf::from(home).join(rest);
+        return home.join(rest);
     }
     let path = PathBuf::from(p);
     if path.is_absolute() {

src/ui/sftp.rs

@@ -310,11 +310,14 @@ fn mode_string(mode: u32) -> String {
     )
 }
 
+/// Where a download lands when nothing else says.
+///
+/// The relative `.` is a last resort rather than a failure because a
+/// download has to be offered somewhere; every other caller of
+/// [`path_display::local_home`](crate::ui::path_display::local_home) wants
+/// the `None` and shows the path whole instead.
 fn local_home() -> PathBuf {
-    std::env::var_os("HOME")
-        .or_else(|| std::env::var_os("USERPROFILE"))
-        .map(PathBuf::from)
-        .unwrap_or_else(|| PathBuf::from("."))
+    crate::ui::path_display::local_home().unwrap_or_else(|| PathBuf::from("."))
 }
 
 fn local_download_dir() -> PathBuf {