Add Format Code function + Format on Save (Qt frontend only)

[aik2mlj]

Hi Ge, this PR is to integrate (and shamelessly promote) my chuckfmt util into miniAudicle.

This adds an Edit → Format Code menu item (Ctrl+Shift+I, which Qt auto-maps to Cmd+Shift+I on macOS) that pipes the active document through an external formatter via stdin → stdout and replaces the buffer with the result. The formatter command is configurable under Preferences → Editing and defaults to chuckfmt; any tool that honors the stdin → stdout contract will work. There's also an opt-in Format on Save checkbox (off by default) that runs the formatter before each save and save-as. If the user doesn't have chuckfmt, they will not run into issues unless they manually trigger formatting or turn on format-on-save.

The format itself is a single action that can be reverted by Ctrl+Z. If the formatter isn't on PATH, exits non-zero, or exceeds a 10-second timeout, the buffer is left untouched and the reason is shown on the status bar. Format-on-save follows the same rule: if formatting fails, the save still proceeds with the unformatted content.

This is the Qt frontend only, which covers the Linux and Windows builds plus make mac-qt for macOS. It does not cover the default Cocoa frontend for macOS yet. I'll implement it once I borrow a mac.

Testing

Install chuckfmt according to its document.

• Format Code reformats a messy .ck; one Ctrl+Z reverts the entire change
• Change formmatter to a nonexisting binary → status bar shows formatter not found: X, buffer unchanged
• Formatter exits non-zero → status bar shows formatter failed: …
• Formatter hangs → formatter timed out after ~10s
• Two open tabs: trigger an error in each; no duplicated status messages
• Enable Format on Save, save a messy file → written file is formatted; disable → plain save
• Format on Save with a broken formatter still writes the file
• Open an example (read-only), Ctrl+S then offers Save As, which writes the formatted content