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/views/HomeView.svelte

@@ -14,6 +14,7 @@
    * start a flow, is `#/entry` (`EntryView`); this screen links to it.
    */
   import PaletteRows from '../components/PaletteRows.svelte';
+  import SavedTrails from '../components/SavedTrails.svelte';
   import { palette } from '../lib/palette.svelte';
   import { buildEntryPalette, type PaletteItem } from '../lib/search-model';
   import { entryHref, fileHref, flowHref, navigate } from '../lib/navigation';
@@ -71,6 +72,13 @@
     </p>
   </div>
 
+  <!-- Above the derived lists on purpose: a walk somebody named and kept is a
+       better place to start than any ranking, when there is one. It draws
+       nothing at all when there is not. -->
+  <div class="saved">
+    <SavedTrails />
+  </div>
+
   {#if entries.sections.length > 0}
     <section class="entries" aria-label="Where to start">
       <div class="entries-h">
@@ -96,6 +104,11 @@
     padding-bottom: 8px;
   }
 
+  .saved {
+    max-width: 800px;
+    padding: 8px 40px 0;
+  }
+
   .entries {
     max-width: 720px;
     padding: 8px 40px 48px;