Wear Catppuccin Mocha, and move Usage to where the provider is

Two changes to the app, plus the one they turned up.

**The theme.** Catppuccin Mocha, copied from dev-updater rather than
shared: wg-app-link is the *link* -- the tunnel, the pinned CA, enrollment
-- and a palette is not that. The two apps looking alike is a preference
rather than a contract, and the moment one wants a different accent a
shared version becomes a thing to fight. dev-updater's ActionTone and its
ANSI table did not come across; nothing here draws a log or a destructive
button yet, and copying a vocabulary with no speakers is how a file starts
lying about what the app does.

**Usage is no longer a global button.** It belongs to the provider, and
the session view is the only place a provider is currently named, so that
is where the control sits -- beside the line that names it, rather than
collected with the app-wide controls where its scope had to be guessed. It
carries the session back with it, so Back returns to that session rather
than dumping the reader on the list.

Its real home is that provider's own settings, which do not exist yet.

**And the bit that only running it could find.** I first subtitled the
usage screen with the session's provider, which on an echo session put
"echo" directly above a card reading "claude" -- Claude's account-wide
numbers labelled as echo's, a claim about echo that nothing measured. The
subtitle is gone; each card names the service that answered, which is the
true scope, and the reason is written where the subtitle was so it does
not get re-added.

Looked at on the emulator rather than read: the palette, the session
header, the usage screen, and Back landing on the session it came from.
ktfmt, compile and Lint clean.

app/androidApp/src/main/kotlin/com/example/aiapp/AppRoot.kt

@@ -26,7 +26,12 @@ private sealed class Screen {
 
     data object Spawn : Screen()
 
-    data object Usage : Screen()
+    /**
+     * Reached from a session rather than from the list, because usage belongs to the provider
+     * running that session and not to the app. It carries the session back with it so Back returns
+     * where it came from -- see [Screen.Session].
+     */
+    data class Usage(val from: SessionSummary) : Screen()
 
     data object Models : Screen()
 
@@ -97,7 +102,6 @@ fun AppRoot(settingsVersion: Int) {
                 reloadToken = reloadToken,
                 onOpen = { screen = Screen.Session(it) },
                 onSpawn = { screen = Screen.Spawn },
-                onUsage = { screen = Screen.Usage },
                 onModels = { screen = Screen.Models },
                 onSetups = { screen = Screen.Setups },
                 onSettings = { screen = Screen.Settings },
@@ -107,6 +111,7 @@ fun AppRoot(settingsVersion: Int) {
                 settings = current,
                 summary = here.summary,
                 onBack = goToList,
+                onUsage = { screen = Screen.Usage(here.summary) },
             )
         is Screen.Spawn ->
             SpawnScreen(
@@ -117,7 +122,13 @@ fun AppRoot(settingsVersion: Int) {
                 },
                 onBack = goToList,
             )
-        is Screen.Usage -> UsageScreen(settings = current, onBack = goToList)
+        is Screen.Usage ->
+            UsageScreen(
+                settings = current,
+                // Back to the session it was opened from, not to the list: this is a step down
+                // from that session, so stepping back is the one thing Back can mean here.
+                onBack = { screen = Screen.Session(here.from) },
+            )
         is Screen.Models -> ModelsScreen(settings = current, onBack = goToList)
         is Screen.Setups -> SetupsScreen(settings = current, onBack = goToList)
         is Screen.Settings ->