Rename binary from procnote-tauri to procnote

[shunichironomura]

Summary

• Add explicit [[bin]] section in src-tauri/Cargo.toml to name the binary procnote instead of procnote-tauri
• Fixes Homebrew cask install which expects the binary at Contents/MacOS/procnote

Context

The Cargo crate name procnote-tauri was being used as the binary name, producing Contents/MacOS/procnote-tauri inside the .app bundle. The Homebrew cask binary stanza expects procnote, causing install to fail with "symlink source is not there".

Test plan

• cargo build -p procnote-tauri produces a binary named procnote
• cargo test --workspace passes
• After next release, brew install --cask shunichironomura/tap/procnote succeeds and procnote --help works

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