fix(submit): allowlist switch_tab values instead of trusting client input

`handle_event("switch_tab", ...)` ran `String.to_existing_atom/1` on
the raw `tab` value from the browser, so a stale, forged, or
mistyped client event with any unknown string crashed the LiveView
with `ArgumentError: not an already existing atom`.

Compares against an explicit `@valid_tabs` allowlist (`:single`,
`:csv`, `:adif`) — known values switch the active tab, anything
else is ignored.

lib/microwaveprop_web/live/submit_live.ex

@@ -54,10 +54,20 @@ defmodule MicrowavepropWeb.SubmitLive do
     end
   end
 
+  @valid_tabs ~w(single csv adif)a
+
   @impl true
   def handle_event("switch_tab", %{"tab" => tab}, socket) do
-    socket = assign(socket, active_tab: String.to_existing_atom(tab))
-    {:noreply, MicrowavepropWeb.LiveStashGuard.stash(socket)}
+    case Enum.find(@valid_tabs, &(Atom.to_string(&1) == tab)) do
+      nil ->
+        # Stale or forged client event — ignore instead of crashing the
+        # LiveView via String.to_existing_atom/1.
+        {:noreply, socket}
+
+      atom ->
+        socket = assign(socket, active_tab: atom)
+        {:noreply, MicrowavepropWeb.LiveStashGuard.stash(socket)}
+    end
   end
 
   def handle_event("validate", %{"contact" => contact_params}, socket) do

test/microwaveprop_web/live/submit_live_test.exs

@@ -43,6 +43,28 @@ defmodule MicrowavepropWeb.SubmitLiveTest do
     end
   end
 
+  describe "switch_tab" do
+    test "ignores an unknown tab string instead of crashing the LiveView", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, ~p"/submit")
+
+      # An unknown tab name used to flow into String.to_existing_atom/1
+      # and bring the LiveView down. We expect the event to be a no-op
+      # and the process to stay alive.
+      render_hook(lv, "switch_tab", %{"tab" => "definitely-not-a-tab-#{System.unique_integer()}"})
+
+      assert Process.alive?(lv.pid)
+    end
+
+    test "honors a known tab name", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, ~p"/submit")
+
+      html = render_hook(lv, "switch_tab", %{"tab" => "csv"})
+
+      # Active tab moved to CSV — the CSV section's heading is now visible.
+      assert html =~ "CSV"
+    end
+  end
+
   describe "validate" do
     test "shows validation errors on change", %{conn: conn} do
       {:ok, lv, _html} = live(conn, ~p"/submit")