fix: support backend account search

[lhiro]

Scope

• Update account list query behavior in mail-worker/src/service/account-service.js.
• Update account list request wrapper in mail-view/src/request/account.js.
• Update account sidebar search in mail-view/src/layout/account/index.vue.
• Update transfer account selector in mail-view/src/views/transfer/index.vue.
• Add a defensive D1 user.lang schema guard in mail-worker/src/service/user-service.js for deployments that have not applied the latest column yet.

State Impact

• Account sidebar no longer derives a filtered list from already-loaded client state; it replaces the account array with backend search results and keeps existing cursor pagination.
• Transfer selector stores the current selected account separately so remote search responses do not drop the selected option unexpectedly.
• Rebuild scope is limited to the account sidebar and transfer selector state.

Network Impact

• GET /account/list now accepts an optional email keyword and searches account email/name on the backend.
• Account sidebar search and transfer account selector now trigger backend requests with a 300ms debounce.
• Existing list size cap and cursor pagination are preserved.
• Request sequence guards discard stale responses to avoid duplicate/out-of-order UI updates.

Startup Impact

• No new startup task.
• Initial account loading still uses the existing /account/list request.
• Expected added startup cost: none.
• Parallelism: unchanged.

Resource Lifecycle

• Search debounce timers are cleared on component unmount in both changed Vue components.
• No new streams, controllers, subscriptions, media resources, or long-lived buffers.

i18n Impact

• No new user-visible copy was added.
• Existing localized labels/messages remain unchanged.

Codegen Impact

• No @riverpod, @freezed, @JsonSerializable, or generated model changes.
• No code generation required.

Analyze Result

• npm run build passed for mail-view.
• npx wrangler deploy --dry-run passed for mail-worker.
• git diff --check passed.

Risk and Rollback Plan

• Risk: backend LIKE search on large account tables may read more rows than the previous first-page-only flow, but it fixes accounts that are outside the first page and keeps the 100-row page cap.
• Rollback: revert this PR to return account search to the previous client-side filtering behavior.

Notes

• Verified the target remote D1 account search condition can find an account beyond the first page, e.g. xigege.edu@v0v.ai.