Redesign the quick-action CTAs: two ways in (study deck vs. homework)

Library empty state ("Your icebox")
- Replace the single "Make your first deck" button with two quick-action
  cards: "Create interactive test" (primary, ice-gradient icon + subtle
  accent border) and "Help me with my homework" (glass card, sparkles
  icon). Stacked on phones, side by side from 640px up; both >=44px tall.

/create
- Replace the "Got homework instead of notes?" banner with a two-card mode
  switcher above "1. Add your notes": "Study Deck & Quizzes" (the current
  view — aria-current, accent tint, ice icon) and "Homework Solver" (a
  shortcut to /homework). Icon-on-top tiles on phones, inline rows from
  640px up; glass tokens + backdrop blur in both themes.

Strings
- New keys in both dictionaries: emptyStateCreateDeck, emptyStateHomework,
  actionStudyDeck, actionHomework. Drop the now-unused homework.entry and
  library.makeFirst keys.

Drive-by AA fix (light mode, /create only)
- The "New folder" dashed button and the difficulty blurbs move from
  frost-500 to frost-600 (4.0:1 -> ~6:1). Dark mode is unchanged.

Service worker: freezmo-v8.

Verified: redesign suite 32/32 (stacked/side-by-side layouts, 360/390/1280,
dark+light, en+cs, >=44px targets, both cards navigate, no overflow, axe
contrast), mobile 49/49, i18n 27/27, desktop regression 73/73. Lint + build
clean.

public/sw.js

@@ -9,7 +9,7 @@
  *
  * Bumping CACHE_VERSION is how a new deploy invalidates old assets.
  */
-const CACHE_VERSION = "freezmo-v7";
+const CACHE_VERSION = "freezmo-v8";
 const SHELL_CACHE = `${CACHE_VERSION}-shell`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;