feat: collections, watchlists, and shareable links (v0.9.0)

Phase 3 completion — Personal Workflow feature set is now complete.

Collections / Watchlists:
- New tables: collections (UUID share_token, slug, public/private) and
  collection_bills (unique bill-per-collection constraint)
- Full CRUD API at /api/collections with bill add/remove endpoints
- Public share endpoint /api/collections/share/{token} (no auth)
- /collections list page with inline create form and delete
- /collections/[id] detail page: inline rename, public toggle,
  copy-share-link, bill search/add/remove
- CollectionPicker bookmark-icon popover on bill detail pages
- Collections nav link in sidebar (auth-required)

Shareable Brief Links:
- share_token UUID column on bill_briefs (backfilled on migration)
- Unified public share router at /api/share (brief + collection)
- /share/brief/[token] — minimal layout, full AIBriefCard, CTAs
- /share/collection/[token] — minimal layout, bill list, CTA
- Share2 button in BriefPanel header row, "Link copied!" flash

AuthGuard: /collections → AUTH_REQUIRED; /share prefix → NO_SHELL_PATHS

Authored-By: Jack Levy
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/components/shared/AuthGuard.tsx

@@ -6,8 +6,8 @@ import { useAuthStore } from "@/stores/authStore";
 import { Sidebar } from "./Sidebar";
 import { MobileHeader } from "./MobileHeader";
 
-const NO_SHELL_PATHS = ["/login", "/register"];
-const AUTH_REQUIRED = ["/following", "/notifications"];
+const NO_SHELL_PATHS = ["/login", "/register", "/share"];
+const AUTH_REQUIRED = ["/following", "/notifications", "/collections"];
 
 export function AuthGuard({ children }: { children: React.ReactNode }) {
   const router = useRouter();
@@ -31,8 +31,8 @@ export function AuthGuard({ children }: { children: React.ReactNode }) {
 
   if (!hydrated) return null;
 
-  // Login/register pages render without the app shell
-  if (NO_SHELL_PATHS.includes(pathname)) {
+  // Login/register/share pages render without the app shell
+  if (NO_SHELL_PATHS.some((p) => pathname.startsWith(p))) {
     return <>{children}</>;
   }