refactor: modularize chat backend, update docs for .m4s segments

frontend/src/style.css

@@ -262,7 +262,7 @@ select {
 .btn {
   border: 1px solid transparent;
   border-radius: var(--radius-1);
-  padding: 0.62rem 0.9rem;
+  padding: 0.2rem 0.5rem;
   font-weight: 650;
   cursor: pointer;
   transition:
@@ -411,14 +411,14 @@ select {
 
 .lv-body {
   min-height: 100vh;
-  width: 100vw;
+  width: 100%;
   height: 100vh;
   padding: 0;
   overflow: hidden;
 }
 
 .viewer-shell {
-  width: 100vw;
+  width: 100%;
   height: 100vh;
   margin: 0;
   display: grid;
@@ -821,7 +821,7 @@ select {
   border: 1px solid var(--line-soft);
   border-radius: var(--radius-2);
   background: var(--panel);
-  padding: var(--space-4);
+  padding: var(--space-2);
   box-shadow: var(--shadow-1);
 }