fix: decrypt local backup data before loading (#1488)

## PR Checklist

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    - [x] Have you added type definitions?
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[^1]: Modifies the behavior of prompting, requesting, or handling
responses from AI models.
[^2]: Over 80% of the code is AI generated.

## Summary

Fixes encrypted local backup loading for account-synced web backups.

The encrypted backup path was fetching the stored crypto key correctly,
but then called `encryptBuffer` while handling `database.risudat`. That
re-encrypted the already encrypted database bytes instead of decrypting
them, causing `decodeRisuSave` to receive invalid data and eventually
fail with fflate errors such as `invalid distance`.

## Related Issues

None.

## Changes

**`src/ts/drive/backuplocal.ts`**
- Import `decryptBuffer`
- Use `decryptBuffer(db, key)` when loading an encrypted
`database.risudat`
- Keep the save path unchanged, where `encryptBuffer(dbData, key)` is
still used to create encrypted backups

## Impact

- Encrypted local backups created from account-synced `risuai.xyz` web
sessions can be loaded correctly.
- Non-encrypted local backups are unaffected because they do not enter
the `encryption.risudat` path.
- Partial local backups are unaffected because they are not encrypted by
this path.
- The save format does not change.

## Additional Notes

Validated with:

- `pnpm check`
- `pnpm test`
- `pnpm build`

Author: kwaroran

src/ts/drive/backuplocal.ts

@@ -6,7 +6,7 @@ import { isTauri } from "src/ts/platform"
 import { decodeRisuSave, encodeRisuSaveLegacy } from "../storage/risuSave";
 import { getDatabase, setDatabaseLite } from "../storage/database.svelte";
 import { relaunch } from "@tauri-apps/plugin-process";
-import { encryptBuffer, sleep } from "../util";
+import { decryptBuffer, encryptBuffer, sleep } from "../util";
 import { hubURL } from "../characterCards";
 import { language } from "src/lang";
 import { getColdStorageItem, listColdDataKeys, setColdStorageItem } from "../process/coldstorage.svelte";
@@ -504,7 +504,7 @@ export function LoadLocalBackup(){
                         if(encryptionMeta.type === 'account' && encryptionMeta.time){
                             try {
                                 const key = (await (await fetch(`https://sv.risuai.xyz/cryptokey?key=${encryptionMeta.time}`)).json()).key
-                                const decrypted = await encryptBuffer(db, key)
+                                const decrypted = await decryptBuffer(db, key)
                                 db = new Uint8Array(decrypted)
                             }
                             catch (e) {