Fail the build when axios is missing (Inertia 3 compatibility)

[shanerbaner82]

Summary

• NativePHP Mobile intercepts `axios` at bundle time and swaps its transport adapter so HTTP requests route through the embedded PHP runtime. Without axios there's nothing to intercept and outgoing requests never reach PHP.
• Through Inertia 2 this was always satisfied: `@inertiajs/vue3` / `@inertiajs/react` pulled axios in transitively. Inertia 3 removed the axios dependency in favour of native `fetch`, leaving fresh Inertia 3 projects with a clean build but an app that can't talk to its own backend.
• This PR makes the Vite plugin throw at `config()` time if axios isn't a declared dependency in the project's `package.json` (or isn't actually present in `node_modules`). The check fires unconditionally — not just under `--mode=ios` — since the same fix applies to any Inertia 3 NativePHP app.

Why check `package.json` and not just the filesystem

A transitive copy of axios can still sit in `node_modules` after the user removes axios from their project (e.g. `@inertiajs/core` itself depends on axios internally). A pure `existsSync` check would pass and the build would silently produce an app that can't reach PHP. Reading the project's `package.json` catches that case — the check fails unless the user has axios declared as a direct dependency of their app.

Error users will see

```
[nativephp/mobile] axios must be declared as a dependency in your package.json.
Inertia 3 removed axios in favour of native fetch; reinstall it so outgoing
requests can be routed through our handler:
npm install axios
Then follow https://inertiajs.com/docs/v3/installation/client-side-setup#using-axios
to tell Inertia to use axios for its client-side requests.
```

Backwards compatibility

• Apps with axios declared already (any Inertia 2 project, or anything using axios directly): no change.
• Fresh Inertia 3 projects: previously built cleanly into an app that couldn't make HTTP requests. Now fails the build with a clear fix.

Paired docs

NativePHP/nativephp.com#370 — adds an aside to `mobile/3/getting-started/development.md` covering the same Inertia 3 requirement next to the existing `nativephpMobile` Vite plugin section.

Test plan

• `npm remove axios` in a vue13 Inertia 3 app, `npm run build` → fails with the error above (a transitive axios in node_modules doesn't mask it).
• `npm install axios`, `npm run build` → succeeds.
• Verify Inertia 3 + NativePHP app can make HTTP requests once axios is installed and Inertia is told to use it per the linked docs.

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