feat(desktop): theme polish, prose chat typography, composer chrome

- DS tokens/midground, Backdrop, scoped scrollbars, typography plugin + prose
- Composer liquid/radius utilities, thread font parity, tool/thinking cues
- File tree label scale, preview flex, thread retry loading + streaming tests

apps/desktop/src/themes/context.tsx

@@ -149,6 +149,11 @@ function lightColors(seed: DesktopTheme, skinName: string): DesktopThemeColors {
     border,
     input: mix('#e2e2e6', accent, 0.18),
     ring: accent,
+    // Brand-accent stroke layer carries the seed's identity color into the
+    // light palette intact (no mix-with-white softening) so DS components
+    // and `bg-midground/N` surfaces still read as branded against white.
+    midground: seed.colors.midground ?? accent,
+    midgroundForeground: readableOn(seed.colors.midground ?? accent),
     destructive: '#b94a3a',
     destructiveForeground: '#ffffff',
     sidebarBackground: mix('#fafafa', accent, 0.05),
@@ -220,6 +225,11 @@ function applyTheme(theme: DesktopTheme, mode: 'light' | 'dark') {
   root.dataset.hermesTheme = skinNameFromTheme(theme, mode)
   root.classList.toggle('dark', rendered === 'dark')
 
+  // Brand-accent stroke layer. Falls back to ring when the theme doesn't
+  // declare its own midground, so existing/custom themes keep working.
+  const midground = c.midground ?? c.ring
+  const midgroundForeground = c.midgroundForeground ?? readableOn(midground)
+
   const vars: Record<string, string> = {
     '--dt-background': c.background,
     '--dt-foreground': c.foreground,
@@ -238,6 +248,8 @@ function applyTheme(theme: DesktopTheme, mode: 'light' | 'dark') {
     '--dt-border': c.border,
     '--dt-input': c.input,
     '--dt-ring': c.ring,
+    '--dt-midground': midground,
+    '--dt-midground-foreground': midgroundForeground,
     '--dt-destructive': c.destructive,
     '--dt-destructive-foreground': c.destructiveForeground,
     '--dt-sidebar-bg': c.sidebarBackground ?? c.background,

apps/desktop/src/themes/presets.ts

@@ -51,6 +51,7 @@ export const nousLightTheme: DesktopTheme = {
     border: '#E3DDCF',
     input: '#D8D1C3',
     ring: '#A0782A',
+    midground: '#A0782A',
     destructive: '#b94a3a',
     destructiveForeground: '#ffffff',
     sidebarBackground: '#F5F2EC',
@@ -74,6 +75,7 @@ export const hermesGoldTheme: DesktopTheme = {
     accent: '#fbf3d4',
     accentForeground: '#5a4310',
     ring: '#d4af37',
+    midground: '#d4af37',
     userBubble: '#f6efd5'
   }
 }
@@ -103,6 +105,7 @@ export const nousTheme: DesktopTheme = {
     border: `color-mix(in srgb, ${NOUS_LENS_BLUE} 22%, transparent)`,
     input: `color-mix(in srgb, ${NOUS_LENS_BLUE} 30%, transparent)`,
     ring: NOUS_LENS_BLUE,
+    midground: NOUS_LENS_BLUE,
     destructive: '#C72E4D',
     destructiveForeground: '#FFFFFF',
     sidebarBackground: `color-mix(in srgb, ${NOUS_LENS_BLUE} 2.5%, #FFFFFF)`,
@@ -140,6 +143,7 @@ export const defaultTheme: DesktopTheme = {
     border: '#1e3232',
     input: '#1e3232',
     ring: '#6bbfb5',
+    midground: '#6bbfb5',
     destructive: '#c0473a',
     destructiveForeground: '#fef2f2',
     sidebarBackground: '#0a1616',
@@ -172,6 +176,7 @@ export const midnightTheme: DesktopTheme = {
     border: '#1e1e52',
     input: '#1e1e52',
     ring: '#8b80e8',
+    midground: '#8b80e8',
     destructive: '#b03060',
     destructiveForeground: '#fef2f2',
     sidebarBackground: '#06061a',
@@ -208,6 +213,7 @@ export const emberTheme: DesktopTheme = {
     border: '#3a1c08',
     input: '#3a1c08',
     ring: '#d97316',
+    midground: '#d97316',
     destructive: '#c43010',
     destructiveForeground: '#fef2f2',
     sidebarBackground: '#100600',
@@ -244,6 +250,7 @@ export const monoTheme: DesktopTheme = {
     border: '#2a2a2a',
     input: '#2a2a2a',
     ring: '#9a9a9a',
+    midground: '#9a9a9a',
     destructive: '#a84040',
     destructiveForeground: '#fef2f2',
     sidebarBackground: '#0a0a0a',
@@ -276,6 +283,7 @@ export const cyberpunkTheme: DesktopTheme = {
     border: '#003000',
     input: '#003000',
     ring: '#00ff41',
+    midground: '#00ff41',
     destructive: '#ff003c',
     destructiveForeground: '#000a00',
     sidebarBackground: '#000600',
@@ -312,6 +320,7 @@ export const slateTheme: DesktopTheme = {
     border: '#30363d',
     input: '#30363d',
     ring: '#58a6ff',
+    midground: '#58a6ff',
     destructive: '#cf4848',
     destructiveForeground: '#fef2f2',
     sidebarBackground: '#090d13',

apps/desktop/src/themes/types.ts

@@ -49,6 +49,17 @@ export interface DesktopThemeColors {
   input: string
   /** Focus ring / primary accent tint. Also `text-ring` in action bars etc. */
   ring: string
+  /**
+   * Brand-accent stroke layer. Distinct from `primary` (CTA fill) — this is
+   * the "this thing is alive / live / signal" color used on focus rings,
+   * streaming cursors, the active session pill, branded scrollbars, and text
+   * selection. Falls back to `ring` when omitted. Aliased to the DS
+   * `--midground` token so `@nous-research/ui` components inherit the
+   * desktop's active theme without further wiring.
+   */
+  midground?: string
+  /** Text on `midground` fills (badges etc). Auto-derived from luminance when omitted. */
+  midgroundForeground?: string
   /** Destructive action (delete, error). */
   destructive: string
   /** Text on destructive. */