feat(local-ai): add guided model tier selection by device capability

Add tiered model presets (Low/Medium/High) with device-aware recommendations
so users can pick a local AI model that fits their machine without editing
raw JSON config. Detect RAM, CPU, GPU via sysinfo crate and recommend a tier.
Persist selection to config.toml, with env var override and graceful
degradation hints on bootstrap failure.

- Rust: presets.rs (tier definitions, recommendation logic), device.rs
  (hardware detection), 3 new RPC methods, env var override, bootstrap hints
- Frontend: tier selector UI in Settings > Local AI Model with device info,
  loading/error states, and "Advanced" toggle for existing controls
- Tests: 7 Rust unit tests + comprehensive JSON-RPC E2E test
- Also fixes pre-existing lint warning in SkillSetupWizard.tsx

Closes #80

.env.example

@@ -88,6 +88,13 @@ OPENHUMAN_PROXY_SCOPE=
 # [optional] Comma-separated services to proxy
 OPENHUMAN_PROXY_SERVICES=
 
+# ---------------------------------------------------------------------------
+# Local AI model tier
+# ---------------------------------------------------------------------------
+# [optional] Override selected model tier: low, medium, high
+# Applies the corresponding preset at config load time (overrides config.toml).
+OPENHUMAN_LOCAL_AI_TIER=
+
 # ---------------------------------------------------------------------------
 # Local AI binary overrides
 # ---------------------------------------------------------------------------