feat(ui): saved trails — a walk you named, kept, and still true after a re-index (CG-60)

Save trail on the trail bar writes the walk to .codegraph/ui/trails/ as one
JSON file, listed on the empty screen and on Entry points above the derived
suggestions, reopened at the symbol you left with the whole path restored.

A hop is stored by qualified name, kind and file — never by node id, which
contains a start line and so changes the first time anybody edits above the
symbol. Every hop is re-resolved against the current index on the way out and
each row says what became of it: still here, moved to another file, now
ambiguous, or gone. A hole is never stitched over: the row opens the longest
run of CONSECUTIVE resolved hops and says which ones those are, because the
trail is a path and a skipped hop would draw a call that does not exist.

This is the first write the viewer makes, and the boundary moved with it:
POST/DELETE answer under /api/ only, must carry X-CodeGraph-UI and
application/json (neither of which a cross-origin form can produce without a
preflight this server answers none of), and --read-only refuses both while
still listing what is there. The blanket "read-only" claim is retired from the
banner, the README, the CLI help and the docs site in favour of the narrower
true one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/src/app.css

@@ -78,11 +78,15 @@ h3 {
 /* ---------- app shell ----------
    Design spec §3.1: top bar 48px / trail bar 34px / main. The grid is on
    index.html's mount host, which App.svelte fills directly (no wrapper — a
-   second #app would duplicate the id). */
+   second #app would duplicate the id).
+
+   The trail row is `auto`, not `--trailbar-h`: the bar keeps that height on
+   its own (see TrailBar.svelte) and grows only while the save-trail form is
+   open. Pinning the row instead would clip the form. */
 #app {
   height: 100vh;
   display: grid;
-  grid-template-rows: var(--topbar-h) var(--trailbar-h) 1fr;
+  grid-template-rows: var(--topbar-h) auto 1fr;
 }
 
 /* ---------- cross-view primitives ---------- */