fix(terminal): init all tabs on load, fix excessive zoom

Use visibility:hidden instead of display:none for inactive terminal tabs
so xterm containers retain their dimensions. This allows all terminals
to initialize independently and prevents fitAddon from miscalculating
cell sizes on zero-height containers.

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

web/src/styles/global.css

@@ -386,6 +386,12 @@ input::placeholder { color: var(--text-disabled); }
 .shell-xterm-instance {
   position: absolute;
   inset: 0;
+  visibility: hidden;
+  pointer-events: none;
+}
+.shell-xterm-instance.active {
+  visibility: visible;
+  pointer-events: auto;
 }
 .shell-xterm-instance .xterm { height: 100%; }