Settings as a screen with two tabs, and fields that cost one line

The settings dialog had outgrown a dialog: it scrolled inside itself,
covered the session it is about, and had nowhere to put a second tab. It
is a screen now, drawn over the session like the file explorer so the
session under it stays composed, with the back gesture to leave it. The
second tab is ProviderScreen itself -- the same composable the machines
tab opens -- so a provider's settings have two ways in and one
implementation.

Every text field in the app goes through LabelledField: the label is a
line above the box rather than a thing floating inside it, the hint says
what leaving it blank means, and the padding is one line's worth.
Material's outlined field spends the height of three lines to hold one,
which on a form of a dozen settings is a screen and a half of scrolling.
The value's own text is unchanged -- the framing was what cost.

A session also gets a system prompt, which for llama.cpp is one entry in
the params table and no app change: it rides in front of the conversation
on every request rather than being recorded as the first thing in it, so
changing it takes effect on the next message. ParamKind::Prose is new
because a paragraph in a one-line box shows six words of itself.

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

@@ -3,12 +3,10 @@ package com.example.aiapp
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.Spacer
-import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
 import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.MaterialTheme
-import androidx.compose.material3.OutlinedTextField
 import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
@@ -146,12 +144,10 @@ fun ProviderLoginDialog(
                                 ) {
                                     Text("Open authorization page")
                                 }
-                                OutlinedTextField(
+                                LabelledField(
+                                    label = "Authorization code",
                                     value = code,
                                     onValueChange = { code = it },
-                                    label = { Text("Authorization code") },
-                                    singleLine = true,
-                                    modifier = Modifier.fillMaxWidth(),
                                 )
                                 current.detail?.let {
                                     Text(it, color = MaterialTheme.colorScheme.error)