ui: consistency sweep — the 1ch rail geometry, Submit thinking, no auto-caps

Owner round 4 (2026-10-05):

- THE RAIL IS THE MISSING CHARACTER: rail + padding = exactly 1ch of
  the mono heading font. 'YOUR THINKING' (13ch) + rail == 'AGENT
  THINKING' (14ch) flush — the chevrons line up vertically, no fixed
  columns. The YOU heading joins the rail (label inside, like the
  thinking rows) — one quote geometry everywhere.
- HEADING REGISTER: YOUR THINKING / AGENT THINKING / YOU all use the
  card eyebrows' small mono caps (text-xs = the 11px rung, font-medium
  uppercase tracking-wide) — the app chrome is JetBrains Mono.
- SUBMIT THINKING: the practice button is now literally the question
  dock's Button (h-9, px-3.5) in one p-1 row with the hint left
  ('↵ submits · ⇧↵ newline') — the keyboard hints are back. Corner
  radii concentric in both docks: shell 16 − 6 = field 10 − 4 = button 6
  (rounded-[10px] fields, rounded-[6px] buttons — the old rounded-xl/12
  was the 'too large' culprit).
- MODEL NAMES: prettyModel no longer auto-capitalizes — names render
  exactly as their providers write them ('glm 5.3 flash').
- Quiz explanations pulled closer (mt-3 → mt-2).

components/chat.tsx

@@ -1259,14 +1259,15 @@ export function Chat({
 										    User text renders plain — the model's LaTeX pipeline is not
 										    applied to learner-authored text ($ stays $, no surprises).
 										    Rendered WITHOUT the Message wrapper — that is the bubble. */
-										<div data-testid="user-message" className="my-1 w-full">
-											<div className="mb-0.5 text-xs font-medium uppercase tracking-wide text-muted-foreground">You</div>
-											<div className="border-l-2 border-muted-foreground/30 pl-3 text-foreground">
-												<div className="lh-prose whitespace-pre-wrap">
+											<div data-testid="user-message" className="my-1 w-full border-l-2 border-muted-foreground/30 pl-[calc(1ch-2px)] font-ui text-xs">
+												{/* The rail GROUPS (owner round 4, 2026-10-05): the YOU heading
+												    sits INSIDE the rail, same mono-xs register and same 1ch
+												    rail as the thinking rows — one quote geometry. */}
+												<div className="font-medium uppercase tracking-wide text-muted-foreground">You</div>
+												<div className="lh-prose whitespace-pre-wrap text-foreground">
 													{m.parts.filter((p) => p.type === "text").map((p) => p.text).join("\n")}
 												</div>
 											</div>
-										</div>
 									) : (
 									<Message from="assistant" className="max-w-full">
 									<MessageContent className="w-full">