Put settings.json where the Android entry point said, not under /

SettingsStore::open read XDG_CONFIG_HOME and HOME itself. Neither
exists on Android, so it resolved to .config/darkroom relative to a
working directory of /, and every settings edit on the tablet failed
with "read-only file system" — the page reported the error and nothing
said why. The doc comment claimed "the same resolution SessionStore
does"; now it is, by calling the same function, which honours the
directory android_main declares and takes the platform's config
directory everywhere else. settings.json sits beside sessions.json on
every platform, as the comment always said it did.

ui/dr-ui/src/settings_store.rs

@@ -25,12 +25,17 @@ pub struct SettingsStore {
 impl SettingsStore {
     /// Open the store at the platform config location.
     ///
-    /// `settings.json` in the platform's config directory — `dr_plat::dirs`
-    /// has the rule per platform — which is the same resolution `SessionStore`
-    /// makes, so the two files sit together and a user backing up one takes
-    /// both.
+    /// **The same resolution `SessionStore` makes**, by calling the same
+    /// function, so `settings.json` sits beside `sessions.json` on every
+    /// platform. That is `dr_plat::dirs`' config directory on a desktop and
+    /// the directory the entry point declared on Android — the second half
+    /// was the bug: this used to read `XDG_CONFIG_HOME` and `HOME` itself,
+    /// neither exists on Android, and the result was `.config/darkroom`
+    /// relative to a working directory of `/`. Every settings edit on a
+    /// tablet failed with "read-only file system", and the page said so
+    /// without saying why.
     pub fn open() -> Self {
-        Self::open_at(dr_plat::base_dir(dr_plat::Base::Config).join("settings.json"))
+        Self::open_at(dr_sync::account::config_dir().join("settings.json"))
     }
 
     /// Open at an explicit path — for tests, and for a non-default location.