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/security/pairing.rs

@@ -184,7 +184,7 @@ fn generate_token() -> String {
     use rand::RngCore;
     let mut bytes = [0u8; 32];
     rand::thread_rng().fill_bytes(&mut bytes);
-    format!("zc_{}", hex::encode(&bytes))
+    format!("zc_{}", hex::encode(bytes))
 }
 
 /// SHA-256 hash a bearer token for storage. Returns lowercase hex.