fix(desktop): two runtime detectors disagree — converge raw __TAURI__ checks on isDesktopRuntime()

## Summary

Two desktop-runtime detectors coexist and disagree:

- `isDesktopRuntime()` (`src/services/runtime.ts:97-113`) — used by ~130 call sites; true on `VITE_DESKTOP_RUNTIME === '1'`, Tauri globals, UA, `tauri:`/`asset:` protocols, `*.tauri.localhost`, or any `https://localhost`/`https://127.0.0.1` origin.
- Raw `'__TAURI_INTERNALS__' in window || '__TAURI__' in window` checks in `src/config/variant.ts:20`, `src/config/basemap.ts:9`, `src/main.ts:487,496`, `src/services/push-notifications.ts:42`, `src/utils/circuit-breaker.ts:55`.

The raw-globals sites behave as *web* during early boot under `npm run desktop:dev` (before the bridge attaches) and in `VITE_DESKTOP_RUNTIME=1` browser builds. Concrete split-brain: `SITE_VARIANT` resolves on the strict check (`variant.ts:20`) while the variant switcher writes the stored variant on the loose check (`src/app/event-handlers.ts:1594`).

## Ask

Converge the raw-globals call sites on `isDesktopRuntime()` (or a deliberate, documented strict variant for the few places that truly need "Tauri IPC available now"), and add a regression test for the desktop:dev early-boot path.

Found during the #5902 parity baseline audit (see `docs/desktop-parity-matrix.md`). Part of #5902.

Start with isDesktopRuntime() in src/services/runtime.ts:97-113, then inspect the raw-global checks in src/config/variant.ts, src/config/basemap.ts, src/main.ts, src/services/push-notifications.ts, and src/utils/circuit-breaker.ts. Add the requested regression coverage for the desktop:dev early-boot path, and consider the work done when these sites consistently use the intended detector without breaking the variant behavior.

Written by the indexing model from the issue text.