fix(totp): only request a code for items that carry a seed

[pyramation]

Summary

Clicking a login spammed the main process with field "seed" not found on item … once a second. ItemDetail guards the polling loop with fields.some(f => f.purpose === 'totp_seed'), but fields holds the previous item's fields until the new item's refresh() resolves — so selecting a login right after a code kept the guard true, and the poller asked for a code with the new item's id.

Two changes, either of which alone would stop the crash:

// renderer: fields can no longer outlive the item they came from
const [loaded, setLoaded] = useState<{ itemId: string; fields: VaultFieldMeta[] }>(…);
const fields = loaded.itemId === item.id ? loaded.fields : [];

// main: absence of a seed is a normal answer, not a raised exception
async totpEntry(itemId: string): Promise<TotpEntry | null> {
  …
  if (!fields.some(f => f.purpose === 'totp_seed')) return null;

totp.code is now Promise<TotpEntry | null>; the renderer clears its interval on a null rather than re-asking every second, and totpList() filters nulls (a kind: 'totp' item whose seed was deleted no longer takes the whole Codes tab down with it). dcrypt vault totp <item> gets the same guard, reporting "<title>" has no one-time-code secret instead of surfacing the PL/pgSQL RAISE.

Link to Devin session: https://app.devin.ai/sessions/04636534e07048089ffb6b78142e12cd
Requested by: @pyramation

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