Mobile Build Support for Sample Demo

**What**

Extend the existing `SampleProject` sample demo with mobile build support for **iOS and Android**, covering two distinct areas:

1. **Mobile input:** `MovementController.cs` uses keyboard input only, and the UI toolkit layouts (`HUD.uxml`, `Landing.uxml`, `Subscribe.uxml`) have no touch controls. Add a virtual joystick and tap-to-interact button so the full subscription gating flow is operable on device.

2. **Local stack networking across devices:** The local development environment runs Anvil on `127.0.0.1:8545` and Ponder on `localhost`. A mobile device on the same LAN cannot reach `localhost` — the RPC URL in `EmbeddedWalletProvider` and the indexer URL in `PonderIndexerProvider` need to point at the host machine's LAN IP. Add runtime config support and document the setup so developers can test the SDK against a local stack from a physical device.

**Why**

The SDK is relevant to mobile game developers but the sample is currently unusable on device — keyboard-only movement means the player cannot move, and `localhost` URLs mean no blockchain or indexer calls succeed. Without both fixes, mobile developers have no working reference point for integrating the SDK into their own projects.

**How**

1. Add `com.unity.inputsystem` to `SampleProject` and migrate `MovementController.cs` to read from the Input System in addition to (not replacing) the existing keyboard path
2. Add an `OnScreenStick` (virtual joystick) and an `OnScreenButton` (interact) to the `HUD.uxml` UI, visible only when `Application.isMobilePlatform` is true
3. Update `GatedAssetTrigger.cs` to respond to the on-screen interact button tap in addition to the existing keyboard prompt
4. Extend `secrets.json` (or add a separate `config.json` in `StreamingAssets/`) with optional `rpcUrl` and `indexerUrl` override fields; when present, `EmbeddedWalletProvider` and `PonderIndexerProvider` use these values at runtime — allowing a developer to point a device at e.g. `http://192.168.x.x:8545` without recompiling
5. Document the LAN networking setup (find host IP, set override URLs, ensure Anvil binds to `0.0.0.0` not `127.0.0.1`) in `docs/local-development.md` (see issue #42)
6. Verify the `SampleProject` compiles cleanly for Android and iOS build targets (no platform-specific compile errors)

**Acceptance Criteria**

- [ ] Sample builds without errors for Android and iOS targets in `SampleProject`
- [ ] Player movement and interaction (including the subscribe flow) are fully operable via touch on a physical device
- [ ] Virtual joystick and interact button are only shown on mobile platforms — PC input paths are unaffected
- [ ] A mobile device on the same LAN can connect to a local Anvil + Ponder stack via the URL override config
- [ ] LAN networking setup is documented in `docs/local-development.md`

**Estimation**

**Dependencies**

#42 (Local Development Docs) — for the LAN networking documentation section.

Start by tracing MovementController.cs, GatedAssetTrigger.cs, HUD.uxml, the two provider classes, and the existing secrets.json or StreamingAssets configuration. Review issue #42 and docs/local-development.md for the networking documentation context, then verify Android and iOS builds and physical-device touch flows; done means the listed acceptance criteria pass without changing PC input behavior.

Written by the indexing model from the issue text.