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/Field.kt

@@ -0,0 +1,126 @@
+package com.example.aiapp
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.interaction.MutableInteractionSource
+import androidx.compose.foundation.interaction.collectIsFocusedAsState
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.text.BasicTextField
+import androidx.compose.foundation.text.KeyboardActions
+import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.material3.LocalTextStyle
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.SolidColor
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.unit.dp
+
+/**
+ * A text field whose label is a line above it rather than a thing floating inside it.
+ *
+ * Every field in this app goes through here, and the reason is vertical space. Material's outlined
+ * field reserves room for a label that animates into its own border and pads the value by half a
+ * line top and bottom, so one setting costs the height of three lines of text to hold one. A form
+ * of ten settings is then a screen and a half of scrolling to read ten short answers.
+ *
+ * What is *not* shrunk is the value itself: it stays at body size, because what is expensive here
+ * is the framing rather than the text, and a field whose contents are smaller than the text beside
+ * it is a field the reader has to lean in to check. See UI_RULES on never shrinking text to fit.
+ *
+ * [hint] is what leaving it blank means, and it goes above the box with the label for the same
+ * reason the label does: inside, it is gone the moment anybody types, which is exactly when a
+ * reader looks back to check what they are overriding.
+ */
+@Composable
+fun LabelledField(
+    label: String,
+    value: String,
+    onValueChange: (String) -> Unit,
+    modifier: Modifier = Modifier,
+    hint: String? = null,
+    enabled: Boolean = true,
+    /**
+     * How many lines the box is, at rest. One for a value; several for prose, where the reader is
+     * writing rather than filling in -- see `ParamKind::Prose`.
+     */
+    lines: Int = 1,
+    keyboardOptions: KeyboardOptions = KeyboardOptions.Default,
+    /** What the keyboard's own action key does, which is usually what the button beside it does. */
+    keyboardActions: KeyboardActions = KeyboardActions.Default,
+) {
+    Column(modifier.fillMaxWidth()) {
+        Text(
+            label,
+            style = MaterialTheme.typography.labelMedium,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+            modifier = Modifier.padding(bottom = 2.dp),
+        )
+        hint?.let {
+            Text(
+                it,
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                modifier = Modifier.padding(bottom = 2.dp),
+            )
+        }
+        FieldBox(value, onValueChange, enabled, lines, keyboardOptions, keyboardActions)
+    }
+}
+
+/** The box itself: the border, the padding, and the text. Shared so the two fields agree. */
+@Composable
+private fun FieldBox(
+    value: String,
+    onValueChange: (String) -> Unit,
+    enabled: Boolean,
+    lines: Int,
+    keyboardOptions: KeyboardOptions,
+    keyboardActions: KeyboardActions,
+) {
+    val interactions = remember { MutableInteractionSource() }
+    val focused by interactions.collectIsFocusedAsState()
+    // The focused border is the accent at the same width as the resting one. Growing it instead
+    // would move the text inside by a pixel on every focus, which is a whole form twitching as the
+    // reader moves down it.
+    val edge =
+        when {
+            !enabled -> MaterialTheme.colorScheme.outlineVariant
+            focused -> MaterialTheme.colorScheme.primary
+            else -> MaterialTheme.colorScheme.outline
+        }
+    val shape = RoundedCornerShape(8.dp)
+    val style =
+        LocalTextStyle.current.merge(
+            TextStyle(
+                color =
+                    if (enabled) MaterialTheme.colorScheme.onSurface
+                    else MaterialTheme.colorScheme.onSurfaceVariant
+            )
+        )
+    BasicTextField(
+        value = value,
+        onValueChange = onValueChange,
+        enabled = enabled,
+        singleLine = lines == 1,
+        minLines = lines,
+        textStyle = style,
+        keyboardOptions = keyboardOptions,
+        keyboardActions = keyboardActions,
+        interactionSource = interactions,
+        cursorBrush = SolidColor(MaterialTheme.colorScheme.primary),
+        modifier =
+            Modifier.fillMaxWidth()
+                .background(MaterialTheme.colorScheme.surfaceContainerHighest, shape)
+                .border(1.dp, edge, shape)
+                .padding(horizontal = 10.dp, vertical = 8.dp),
+        decorationBox = { field -> Box { field() } },
+    )
+}