Terminal: a shell on the agent chat's computer, before the first message too

The Terminal panel opens a shell on the device an agent chat works on, in its
folder: for a new agent chat as soon as the device and folder are chosen (the
server's /api/v2/devices/{device}/terminal/ws?directory=, which the chat's first
message takes over -- the same socket carries on), and for an open agent chat
from the header (/api/v2/chats/{chat}/terminal/ws, the web page's shell).
Offered where GET /devices says the device opens a terminal.

The socket is Rust's (src-tauri/src/terminal.rs, tungstenite on the rustls
ureq already brings): the bearer token goes in the handshake and never reaches
the page, which gets the frames as `terminal` events and sends keys, sizes and
close through commands. 4401 signs the app out as a 401 does.

xterm and its fit addon are the server's vendored build, pinned through
fetch-vendor.sh like CodeMirror and loaded on the first open. Send to chat puts
the selection, the last marked command and its output, or the screen into the
composer, never sent. The terminal and the activity panel take turns in the
panel column. Continue in a terminal stays in the chat's menu.

tests/ui: seven B3 cases with a mocked socket, at 1280 and 390 px.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/fetch-vendor.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
-# Put the server's vendored CodeMirror build where the app's own pages read it from:
+# Put the server's vendored CodeMirror build and xterm where the app's own pages read them from:
 #   ui/vendor/*.mjs   (copies of the server's static/vendor/cm-*.mjs and lezer-*.mjs)
+#   ui/vendor/xterm.js, xterm.css, xterm-addon-fit.js   (the terminal panel's, copied unchanged:
+#                     classic scripts the panel loads when it is first opened)
 #
 #   scripts/fetch-vendor.sh <server-ref>      copy the modules at that ref (a tag such as
 #                                             v2.0.0-beta.1, or a branch) and write vendor.lock.json
@@ -47,8 +49,8 @@ if [ $# -eq 0 ]; then
       echo "fetch-vendor: $OUT/$name does not match $LOCK (server $ref)" >&2
       bad=1
     fi
-  done < <(sed -n -E 's/^ *"([a-z0-9-]+\.mjs)": *\{"server": *"[0-9a-f]{64}", *"copy": *"([0-9a-f]{64})"\}.*/\1\t\2/p' "$LOCK")
-  for f in "$OUT"/*.mjs; do
+  done < <(sed -n -E 's/^ *"([a-z0-9-]+\.(mjs|js|css))": *\{"server": *"[0-9a-f]{64}", *"copy": *"([0-9a-f]{64})"\}.*/\1\t\3/p' "$LOCK")
+  for f in "$OUT"/*; do
     grep -q "\"$(basename "$f")\"" "$LOCK" || { echo "fetch-vendor: $f is not in $LOCK" >&2; bad=1; }
   done
   [ "$count" -gt 0 ] || { echo "fetch-vendor: $LOCK lists no file" >&2; exit 1; }
@@ -87,6 +89,16 @@ for name in $names; do
   fi
   entries+=("$(printf '    "%s": {"server": "%s", "copy": "%s"}' "$name" "$have" "$(sha "$work/out/$name")")")
 done
+# xterm and its fit addon, for the terminal panel: copied as they are, each checked against the
+# server's pin like the modules.
+for name in xterm.js xterm.css xterm-addon-fit.js; do
+  git -C "$SERVER" show "$commit:$SOURCE/$name" > "$work/out/$name" \
+    || { echo "fetch-vendor: $name is not in $SOURCE at $REF" >&2; exit 1; }
+  pinned="$(python3 -c 'import json,sys; print(json.load(open(sys.argv[1])).get(sys.argv[2], {}).get("sha256", ""))' "$work/server-lock.json" "$name")"
+  have="$(sha "$work/out/$name")"
+  [ "$pinned" = "$have" ] || { echo "fetch-vendor: $name does not match the server's pin ($have, pinned $pinned)" >&2; exit 1; }
+  entries+=("$(printf '    "%s": {"server": "%s", "copy": "%s"}' "$name" "$have" "$have")")
+done
 # The polyfill stub, from the server's own scripts (it is not a download, so it has no pin there).
 stub="src/lembas/web/static/js/node-process.mjs"
 git -C "$SERVER" show "$commit:$stub" > "$work/out/node-process.mjs" \
@@ -101,7 +113,7 @@ done
 
 rm -rf "$OUT"
 mkdir -p "$OUT"
-cp "$work"/out/*.mjs "$OUT/"
+cp "$work"/out/*.mjs "$work"/out/xterm.js "$work"/out/xterm.css "$work"/out/xterm-addon-fit.js "$OUT/"
 {
   printf '{\n  "tag": "%s",\n  "commit": "%s",\n  "files": {\n' "$REF" "$commit"
   last=$((${#entries[@]} - 1))