feat(learner): web shell, library, lesson views (LessonView), decks, code files, exercises, notes, account and admin pages

Tasks 10-12 of the WP-A plan in one commit (the route modules import each other):
- header extras portalled into the Phase 0 header: course switcher, search, Reading/Work, account
- course frame with outline sidebar (mobile overlay, Escape closes), progress, gating locks
- library: featured course, continue row, New and collection rows, course list, filters, search
- LessonView: text (track switcher, folds, ordered hint tiers), video (resume, done near the end,
  timestamp notes, transcript), companion decks rendered with pdf.js (thumbs, arrows, fullscreen,
  per-slide notes), code files (tree, tabs, copy, zip, command), exercises (Web Worker runner or
  local command with optional pasted evidence), floating action bar
- notes list, account (email/password, providers, SSO by email) and owner admin pages
- pdf.js worker served from /api/learner/pdf.worker.mjs
- browser entry @stc/feature-learner/web (the package index also carries the server half)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/features.ts

@@ -1,4 +1,5 @@
 import type { WebFeature } from "@stc/contracts";
+import { learnerWeb } from "@stc/feature-learner/web";
 
 // Phase 1 packages append one import line each and push into this list.
-export const webFeatures: WebFeature[] = [];
+export const webFeatures: WebFeature[] = [learnerWeb];