Keep several saves, with migration, atomic writes and import previews (Closes #37)

Savegames now lists independent save slots made with the loaded ROM:
new game, continue, rename, duplicate, export and delete. The native
database moves to IndexedDB version 2; the singleton save becomes
"Save 1" inside the upgrade transaction, so it migrates exactly once and
the old store is left untouched. Each write adds a generation and drops
older ones in one transaction, keeping two records with CRC-32
checksums: a damaged newest record falls back to the previous one.
Slots and records from a newer player are kept and never written.

Imports are checked in a scratch world before anything changes and
previewed with the trainer, progress, saved time, game data version and
the slots of the same trainer; they go into a new save or replace one,
whose previous record stays recoverable.

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

tests/quest-browser.spec.ts

@@ -99,8 +99,8 @@ test("Mr. Pokémon's ROM scene grants the Egg and Pokédex, heals and survives r
   expect(state.scenes).toMatchObject({ "26:10": 1, "26:3": 1, "24:5": 3 });
   expect(state.party[0].hp).toBe(state.party[0].stats[0]);
   expect(state.specialPhoneCall).toBe(2); // SPECIALCALL_ROBBED
-  page.once("dialog", (dialog) => dialog.accept());
   await page.locator("#save-file").setInputFiles(path);
+  await page.locator("#import-current").click();
   await expect(page.locator("#status")).toContainText("Backup restored");
   await page.reload();
   await page.locator("#rom-file").setInputFiles(romPath);