refactor: remove unused greet command

[whitelonng]

Summary

Removes the unused greet command, which was a placeholder from Tauri scaffolding that is not used anywhere in the application.

Changes

• Remove greet() function from src-tauri/src/lib.rs
• Remove greet from invoke_handler registration

Verification

• ✅ No frontend code calls invoke('greet', ...)
• ✅ Command was never part of the application's actual functionality
• ✅ 186 Rust unit tests pass
• ✅ TypeScript type check passes

Note on Issue #9 (reqwest::blocking)

After investigation, reqwest::blocking is actively used by the crawler.rs module for the crawl_fanqie_article command:

// src-tauri/src/crawler.rs
use reqwest::blocking::Client;

pub fn crawl_fanqie_article(...) -> Result<String, String> {
    let client = Client::builder()...
    // Used for synchronous HTTP requests
}

The blocking feature cannot be removed without:

1. Migrating the entire crawler module to async
2. Converting crawl_fanqie_article command to async
3. Updating all HTTP requests in crawler to use async reqwest::Client

This is a larger refactor beyond the scope of removing unused code. Issue #9 may be:

• A false positive (blocking feature is actually used)
• A suggestion for future async migration (not a bug)

Test Plan

• ✅ All Rust unit tests pass
• ✅ TypeScript compilation passes
• Manual verification: no references to greet in frontend code