Add kids news channel; document per-source language filter

- New kids channel (/kids) with theme color, surfaced in the web UI
  and routed to by BBC Newsround, DOGO News, Science News Explores,
  and NASA for Students in the example config.
- README and config.example.toml now mention the optional
  per-source `language` filter added in the previous commit.

internal/web/static/css/input.css

@@ -76,6 +76,7 @@ html[data-phase="night"] {
   .bg-theme-music     { background-color: #b079d6; }
   .bg-theme-anime     { background-color: #ec5e8a; }
   .bg-theme-foss      { background-color: #d97706; }
+  .bg-theme-kids      { background-color: #14b8a6; }
 
   .text-theme-gaming    { color: #2d8a5a; }
   .text-theme-tech      { color: #2f7fb8; }
@@ -84,6 +85,7 @@ html[data-phase="night"] {
   .text-theme-music     { color: #8a4fb8; }
   .text-theme-anime     { color: #c33a6a; }
   .text-theme-foss      { color: #b8530a; }
+  .text-theme-kids      { color: #0f766e; }
 
   .decoration-theme-gaming    { text-decoration-color: #4caf7d; }
   .decoration-theme-tech      { text-decoration-color: #5aa9e6; }
@@ -92,6 +94,7 @@ html[data-phase="night"] {
   .decoration-theme-music     { text-decoration-color: #b079d6; }
   .decoration-theme-anime     { text-decoration-color: #ec5e8a; }
   .decoration-theme-foss      { text-decoration-color: #d97706; }
+  .decoration-theme-kids      { text-decoration-color: #14b8a6; }
 
   .border-theme-gaming    { border-color: #4caf7d; }
   .border-theme-tech      { border-color: #5aa9e6; }
@@ -100,6 +103,7 @@ html[data-phase="night"] {
   .border-theme-music     { border-color: #b079d6; }
   .border-theme-anime     { border-color: #ec5e8a; }
   .border-theme-foss      { border-color: #d97706; }
+  .border-theme-kids      { border-color: #14b8a6; }
 
   /* "Posted to Matrix" glow — soft pulsing aura in the channel's theme color. */
   .glow-theme-gaming   { box-shadow: 0 0 0 2px rgba(76,175,125,0.35), 0 0 24px 4px rgba(76,175,125,0.45); animation: pete-glow-pulse 2.4s ease-in-out infinite; }
@@ -109,6 +113,7 @@ html[data-phase="night"] {
   .glow-theme-music    { box-shadow: 0 0 0 2px rgba(176,121,214,0.35), 0 0 24px 4px rgba(176,121,214,0.45); animation: pete-glow-pulse 2.4s ease-in-out infinite; }
   .glow-theme-anime    { box-shadow: 0 0 0 2px rgba(236,94,138,0.35), 0 0 24px 4px rgba(236,94,138,0.45); animation: pete-glow-pulse 2.4s ease-in-out infinite; }
   .glow-theme-foss     { box-shadow: 0 0 0 2px rgba(217,119,6,0.35), 0 0 24px 4px rgba(217,119,6,0.45); animation: pete-glow-pulse 2.4s ease-in-out infinite; }
+  .glow-theme-kids     { box-shadow: 0 0 0 2px rgba(20,184,166,0.35), 0 0 24px 4px rgba(20,184,166,0.45); animation: pete-glow-pulse 2.4s ease-in-out infinite; }
 
   @keyframes pete-glow-pulse {
     0%, 100% { filter: brightness(1); }