fix: add missing elements migration and harden unlockElement action

- Add missing elements to mana store migration: when loading an old save
  that doesn't have all elements from ELEMENTS (e.g. saves from before
  composite/exotic elements were added), the migration now creates the
  missing elements with proper default state
- Harden unlockElement action to handle the case where an element doesn't
  exist in the store (creates a valid element state instead of corrupting
  with { unlocked: true } missing current/max/baseMax)
- Add regression tests (14 tests) for Earth element unlock and migration
  scenarios including old saves with missing elements

Fixes #338, #339

docs/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-06-09T17:09:03.568Z",
+    "generated": "2026-06-10T07:56:29.402Z",
     "description": "Import dependency graph for src/lib/game. Keys are files, values are arrays of files they import.",
     "usage": "To find what a file affects, search for its path in the VALUES. To find what a file depends on, look at its KEY entry."
   },