#434 Add optional unified Strata Manager

描述

Summary

Adds an optional Strata Manager that turns the existing Strata web UI into a unified local control surface:

Manager · Chat · Monitor · About

The Manager runs as a persistent local control server on 127.0.0.1:8275, while the normal Strata model server remains on its
existing port (usually 8080).

This keeps model configuration and lifecycle controls available even while Strata itself is stopped or restarting, without changing
the existing START-HERE / SETUP workflows.

What it adds

The new Manager tab supports:

- Discovering and selecting installed Strata models
- Context size presets and custom context values
- Vision: Off / GPU / CPU
- Low-RAM mode
- KV cache settings
- GPU selection
- Host / port / API key settings
- Existing custom GGUF selection
- Start / Stop / Restart
- Force Stop fallback where appropriate
- Installed-model overview
- Server logs
- Local CPU / RAM / GPU information

Configuration continues to use the existing setup.py semantics and the same strata-*.json files. The Manager does not introduce a
second configuration system.

Unified web UI

The existing Strata frontend is reused rather than embedded as a separate site.

The result is one native interface:

```text
  Strata

  [ Manager ] [ Chat ] [ Monitor ] [ About ]
```