fix: persist model name to .env so dotted names survive restart (#426)

* fix: persist model name to .env so dotted names survive restart (#400)

The setup wizard saved selected_model to the DB but not to .env.
Since Config::from_env_with_toml() runs before the DB connects, the
model name was lost on restart -- backends fell back to hardcoded
defaults, truncating names like "llama3.2" to "llama3".

- Add LlmBackend::model_env_var() as single source of truth for the
  backend-to-env-var mapping
- Write the model env var in write_bootstrap_env() using the new method
- Add selected_model fallback to all 6 backends (was missing from
  OpenAI, Anthropic, Ollama, and Tinfoil)

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* refactor: extract resolve_model() helper to reduce duplication

Address review feedback: the env → settings → default model resolution
pattern was repeated across all 6 backends.  Centralise it in a single
LlmConfig::resolve_model() helper.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/setup/wizard.rs

@@ -2040,6 +2040,20 @@ impl SetupWizard {
             env_vars.push(("OLLAMA_BASE_URL", url.clone()));
         }
 
+        // Model name: same chicken-and-egg — Config::from_env() resolves the
+        // model before the DB is connected, so we must persist it to .env.
+        // Write the backend-specific env var so the correct resolution path
+        // picks it up.
+        if let Some(ref model) = self.settings.selected_model {
+            let backend: crate::config::LlmBackend = self
+                .settings
+                .llm_backend
+                .as_deref()
+                .and_then(|s| s.parse().ok())
+                .unwrap_or_default();
+            env_vars.push((backend.model_env_var(), model.clone()));
+        }
+
         // Preserve NEARAI_API_KEY if present (set by API key auth flow)
         if let Ok(api_key) = std::env::var("NEARAI_API_KEY")
             && !api_key.is_empty()