fix: resolve all clippy warnings, formatting, and Mistral endpoint

- Fix Mistral provider base URL (missing /v1 prefix caused 404s)
- Resolve 55 clippy warnings across 28 warning types
- Apply cargo fmt to 44 formatting violations
- Remove unused imports (process_message, MultiObserver, VerboseObserver,
  ChatResponse, ToolCall, Path, TempDir)
- Replace format!+push_str with write! macro
- Fix unchecked Duration subtraction, redundant closures, clamp patterns
- Declare missing feature flags (sandbox-landlock, sandbox-bubblewrap,
  browser-native) in Cargo.toml
- Derive Default where manual impls were redundant
- Add separators to long numeric literals (115200 → 115_200)
- Restructure unreachable code in arduino_flash platform branches

All 1,500 tests pass. Zero clippy warnings. Clean formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/onboard/wizard.rs

@@ -2271,14 +2271,11 @@ fn setup_memory() -> Result<MemoryConfig> {
     let backend = backend_key_from_choice(choice);
     let profile = memory_backend_profile(backend);
 
-    let auto_save = if !profile.auto_save_default {
-        false
-    } else {
-        Confirm::new()
+    let auto_save = profile.auto_save_default
+        && Confirm::new()
             .with_prompt("  Auto-save conversations to memory?")
             .default(true)
-            .interact()?
-    };
+            .interact()?;
 
     println!(
         "  {} Memory: {} (auto-save: {})",