feat(registry): add playwright capability group (6 TS browser fns)

New domain `browser` under frontend/functions/ with 6 Playwright helpers:
- pw_launch_browser: chromium + context + page bootstrap with storageState
  support and baseUrl navigation.
- pw_kanban_login: authenticates a Page against /api/auth/login; sets the
  kanban_session cookie via shared storageState; verifies login page no
  longer visible after navigation.
- pw_drag_drop: human-like pointer drag (mousedown + activateOffset +
  stepped move + mouseup) compatible with @dnd-kit/core's 8px activation
  threshold; supports hoverMs for time-based dropzones.
- pw_keyboard_sequence: ordered focus/type/press/wait steps for scripting
  realistic input flows (typing then arrow-key navigating autocompletes).
- pw_wait_predicate: thin wrapper over page.waitForFunction with friendlier
  defaults and custom error messages.
- pw_assert_class: poll-based assertion that a Locator has/lacks a CSS
  class within a timeout; useful for visual-state checks.

Each function ships with vitest tests (5-8 cases each) covering both happy
and error paths, plus self-documenting .md (Ejemplo + Cuando usarla +
Gotchas + frontmatter with params/output schema).

Adds frontend/functions/package.json with `"type": "module"` so consumers
can ESM-import the .ts files from anywhere in the registry (Playwright's
tsx loader respects nearest package.json).

Capability page docs/capabilities/playwright.md documents the group with
a canonical end-to-end example, frontiers, prerequisites, and gotchas.
Index updated.

First consumer (issue 0088): apps/kanban requester-input.spec.ts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/functions/browser/pw_kanban_login.md

@@ -0,0 +1,53 @@
+---
+name: pw_kanban_login
+kind: function
+lang: ts
+domain: browser
+version: "1.0.0"
+purity: impure
+signature: "async (page: Page, opts: PwLoginOptions) => Promise<void>"
+description: "Authenticates a Playwright Page against the kanban backend via POST /api/login. Sets session cookie and navigates to root. Throws on failure or if login page remains visible."
+tags: [playwright, e2e, browser, kanban]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: []
+params:
+  - name: page
+    desc: "Playwright Page (typically from pw_launch_browser). The page context shares storageState with page.request, so the Set-Cookie response propagates automatically."
+  - name: opts
+    desc: "{username, password, baseUrl?}. baseUrl defaults to the origin derived from page.url() — so the page must already be navigated to the target host before calling this function."
+output: "void; throws on failed login (HTTP >= 400) or if login page selectors remain visible after navigation. After resolve, page is on root (baseUrl + '/') and the kanban_session cookie is active."
+tested: true
+tests:
+  - "happy path"
+  - "rejects 401"
+  - "rejects when login page still visible"
+test_file_path: "frontend/functions/browser/pw_kanban_login.test.ts"
+file_path: "frontend/functions/browser/pw_kanban_login.ts"
+---
+
+## Ejemplo
+
+```typescript
+import { pw_launch_browser } from "@fn_library/../browser/pw_launch_browser";
+import { pw_kanban_login } from "@fn_library/../browser/pw_kanban_login";
+
+const { page, close } = await pw_launch_browser({ baseUrl: "http://localhost:5180" });
+await pw_kanban_login(page, { username: "egutierrez", password: "..." });
+// page is now on http://localhost:5180/ with kanban_session cookie active
+await close();
+```
+
+## Cuando usarla
+
+Usala al inicio de cualquier test Playwright o script e2e contra el kanban. Despues de esta llamada el `page` esta autenticado y puede acceder a rutas protegidas sin redireccion al login.
+
+## Gotchas
+
+- `page.url()` debe devolver una URL valida con origin antes de llamar a esta funcion si no se pasa `baseUrl`. Si la pagina no ha navegado todavia (`about:blank`), pasar `baseUrl` explicitamente.
+- La funcion asume que `page.request` comparte el contexto de cookies con `page` (comportamiento por defecto de Playwright). Si usas un `APIRequestContext` separado, la cookie no se propagara automaticamente.
+- La comprobacion post-login busca texto literal "Login" o "Iniciar sesion". Si el backend usa otro texto en el boton, la comprobacion pasara aunque la autenticacion haya fallado silenciosamente — en ese caso ampliar los selectores de sanidad.
+- Espera `networkidle` tras `goto`: si la app lanza requests continuos (polling, WebSocket upgrade), `networkidle` puede tardar o no resolverse. Considerar `page.waitForURL` como alternativa en esos casos.