When the desktop chat UI is served as a plain web page (no
window.__TAURI_INTERNALS__), get_identity() has no way to succeed, so
useIdentityQuery's queryFn always rejects. With the default retry
config, the single configured retry can call the query-core retryer's
pause() path if the window isn't focused at that exact moment
(canContinue() requires focusManager.isFocused() regardless of
networkMode) — leaving the query stuck on fetchStatus "paused"
forever instead of ever settling to "error". Any consumer gating UI
on this query's settled status (success/error) then hangs
indefinitely, since neither state is ever reached.

- useIdentityQuery: retry: false — a missing native identity backend
  can't be fixed by retrying, so there's no reason to ever enter the
  retry/pause branch.
- tauriIdentity.ts: guard get_identity/get_nsec/import_identity/
  persist_current_identity/sign_out with isTauri(), matching the
  guard convention already used elsewhere in this codebase (e.g.
  audioWorklet.ts), so a missing runtime produces a clean, identifiable
  Error instead of a raw "Cannot read properties of undefined
  (reading 'invoke')" TypeError surfaced from deep inside invoke().

Covers the actual failure mode this PR fixes: get_identity and its
siblings must never reach invokeTauri when there is no Tauri runtime.
Without this guard the query hung on fetchStatus "paused" instead of
settling to "error" — these tests assert the guard short-circuits
before any native call is attempted, and that it doesn't when Tauri
is available.

The "Tauri available" happy-path case lives in its own file
(tauriIdentity.available.test.mjs) rather than alongside the
"unavailable" cases: node's per-process ESM module cache means mixing
an isTauri: () => true mock with an isTauri: () => false mock in the
same test file causes a later import of tauriIdentity.ts to return
the first test's cached module instance instead of re-evaluating
against the new mock. node --test already runs each matched file in
its own process, which sidesteps that.

Requires --experimental-test-module-mocks (added to the test script)
for node:test's t.mock.module — available in the project's pinned
Node version (22.22.3) but not previously enabled.