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.

AGENTS.md

@@ -180,6 +180,19 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   state and thread id, and reads subscription limits through the same CLI
   protocol.
 - `app/` — the Compose app, package `com.example.aiapp`, label "AI Sessions".
+  **Every text field is `LabelledField`** (`Field.kt`): the label is a line
+  above the box rather than a thing floating inside it, 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. What is *not* shrunk is the value -- the framing is what was
+  expensive. A field's `hint` is what leaving it blank means, above the box
+  with the label, because inside it is gone the moment anybody types.
+  **Session settings is a screen with two tabs** (`SessionSettingsScreen.kt`),
+  drawn over the session like the file explorer so the session stays composed.
+  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; it takes `onBack = null` there, since the screen around it
+  has one.
   `AppRoot.kt` is the navigation `when`; `SidePanels.kt` the one drag that
   slides the whole main screen over a session from the left (`MainPanel.kt`)
   and what it has running beside the turn -- its background tasks over its