feat: add codex / lore book with 364 entries

Implements a full in-game Codex panel that tracks lore discovery across
seven categories: bosses (72), quests (95), zones (18), equipment (65),
adventurer tiers (32), upgrades (57), and prestige upgrades (25).

Lore entries unlock automatically as players progress — existing
completions are retroactively and silently added on first load. New
discoveries trigger a toast notification and badge counter on the
Codex tab.

IDEAS.md

@@ -22,7 +22,7 @@ A running list of planned features and content additions. Strike through items a
 
 - [x] **Equipment set bonuses** — Group existing equipment into named sets (e.g. "Shadow Infiltrator"). Wearing 2/3/4 pieces of a set grants escalating bonuses. Adds strategic depth without requiring lots of new items.
 
-- [ ] **The Codex / Lore Book** — Defeating bosses and completing quests unlocks lore entries about the world. Pure flavour, but gives the world depth and a collection mechanic. Show a ✨ notification when new lore unlocks.
+- [x] **The Codex / Lore Book** — Defeating bosses and completing quests unlocks lore entries about the world. Pure flavour, but gives the world depth and a collection mechanic. Show a ✨ notification when new lore unlocks.
 
 - [x] **Milestone prestige bonuses** — Every 5th prestige, earn a free prestige upgrade or a large runestone windfall. Gives players mini-goals within the prestige loop.
 
@@ -45,5 +45,5 @@ A running list of planned features and content additions. Strike through items a
 5. ~~Milestone prestige bonuses~~ ✅
 6. ~~Equipment set bonuses~~ ✅
 7. ~~Auto-prestige toggle~~ ✅
-8. The Codex / Lore Book (flavour, lower priority)
+8. ~~The Codex / Lore Book~~ ✅
 9. Second prestige layer / Transcendence (big feature, save for later)