Fix Unicode-safe table truncation

[BinaryHusk]

Summary

• Fix table truncation to count Unicode scalar values instead of UTF-8 bytes.
• Prevent panics when repository names or file paths contain multibyte characters.
• Add regression tests for Chinese paths and repository names.

Reproduction log

Before this change, scanning DeepSeek keys could panic while rendering results if a repository file path contained multibyte characters:

❯ ./target/release/keyhunter scan -p deepseek

    ╦╔═╔═╗╦ ╦╦ ╦╦ ╦╔╗╔╔╦╗╔═╗╦═╗
    ╠╩╗║╣ ╚╦╝╠═╣║ ║║║║ ║ ║╣ ╠╦╝
    ╩ ╩╚═╝ ╩ ╩ ╩╚═╝╝╚╝ ╩ ╚═╝╩╚═

    Fast GitHub API Key Scanner
    ────────────────────────────

🔍 Scanning 1 provider(s), 6 queries total
ℹ Press Ctrl+C to stop and save results

→ Provider: DeepSeek (DeepSeek API Key)
    ⟩ Query 1/6: DEEPSEEK_API_KEY
      ⠋ Searching...
      ↳ 3 keys found
    ⟩ Query 2/6: DEEPSEEK_API_KEY filename:.env
      ⠋ Searching...
      ↳ 12 keys found
    ⟩ Query 3/6: DEEPSEEK_KEY
      ⠋ Searching...
      ↳ 2 keys found
    ⟩ Query 4/6: DEEPSEEK_KEY filename:.env
      ⠋ Searching...
      ↳ 9 keys found
    ⟩ Query 5/6: deepseek_api
      ⠋ Searching...
      ↳ 3 keys found
    ⟩ Query 6/6: deepseek_api filename:.env
      ⠋ Searching...
      ↳ 15 keys found
    ✓ Running total: 33 unique keys


════════════════════════════════════════════════════════════════════════════════
🔑 Found 33 potential leaked keys


thread 'main' (1115095) panicked at src/scanner.rs:844:28:
byte index 4 is not a char boundary; it is inside '州' (bytes 3..6) of `瀛州纪/.env.backup`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Testing

• cargo test
• cargo build --release
• ./target/release/keyhunter scan -p deepseek