refactor: complete Rust 2024 let-chain migration

Migrate all remaining collapsible_if patterns to Rust 2024 let-chain
syntax across the entire codebase. This modernizes conditional logic
by replacing nested if statements with single-level expressions using
the && operator with let patterns.

Changes:
- storage.rs: 2 let-chain conversions (database dir creation, legacy archiving)
- session.rs: 3 let-chain conversions (empty content check, ledger dir creation, consent flow)
- ollama.rs: 8 let-chain conversions (socket parsing, cloud validation, model caching, capabilities)
- main.rs: 2 let-chain conversions (API key validation, provider enablement)
- owlen-tui: ~50 let-chain conversions across app/mod.rs, chat_app.rs, ui.rs, highlight.rs, and state modules

Test fixes:
- prompt_server.rs: Add missing .await on async RemoteMcpClient::new_with_config
- presets.rs, prompt_server.rs: Add missing rpc_timeout_secs field to McpServerConfig
- file_write.rs: Update error assertion to accept new "escapes workspace boundary" message

Verification:
- cargo build --all: ✅ succeeds
- cargo clippy --all -- -D clippy::collapsible_if: ✅ zero warnings
- cargo test --all: ✅ 109+ tests pass

Net result: -46 lines of code, improved readability and maintainability.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

crates/owlen-core/tests/file_write.rs

@@ -60,7 +60,9 @@ async fn write_outside_root_is_rejected() {
     // The server returns a Network error with path traversal message
     let err_str = format!("{err}");
     assert!(
-        err_str.contains("path traversal") || err_str.contains("Path traversal"),
+        err_str.contains("path traversal")
+            || err_str.contains("Path traversal")
+            || err_str.contains("escapes workspace boundary"),
         "Expected path traversal error, got: {}",
         err_str
     );