Add per-row session delete + Reset all data; fix viewer URL fallback

UX gaps that surfaced while running the demo:

- ViewerShell hardcoded `/fixtures/pdfs/<title>` for the PDF URL,
  ignoring the `document.uri` blob URL that uploaded PDFs carry. The
  viewer either 404'd or — worse — silently served a fixture whose
  filename happened to collide. Prefer document.uri when present.

- SessionMenu only let you delete the *active* session. Added a small
  per-row "✕" button next to every session in the Switch-to list so a
  user can drop a session's data without first switching to it. Same
  click-to-confirm pattern as the existing Delete action.

- Added a "Reset all data…" affordance in both the SessionMenu and the
  empty-state landing. Calls a new `clearAllSessionData()` helper that
  wipes every `citation-evidence:*` key from localStorage, then forces
  a reload so all in-memory caches start fresh.

- `attachSessionPersister.writeOnDelete` was leaking the per-session
  `active-document-id:v1` key on every session delete. Now removed
  alongside the engine snapshot key.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/work/ViewerShell.tsx

@@ -44,6 +44,13 @@ export function ViewerShell() {
 
   const fileUrl = useMemo(() => {
     if (!document) return null;
+    // CE-WP-0005: uploads + sample sessions stash a `blob:` URL on
+    // `document.uri` via the per-session `PdfByteStore`. Prefer that
+    // over the legacy fixture-path fallback so user uploads don't get
+    // resolved against `/fixtures/pdfs/` (which would either 404 or —
+    // worse — silently return the wrong file when the filename happens
+    // to collide with a bundled fixture).
+    if (document.uri) return document.uri;
     const titleOrId = document.title ?? document.id;
     return `/fixtures/pdfs/${encodeURIComponent(titleOrId)}`;
   }, [document]);