feat: add customizable UI styles for menus and messages

[sei40kr]

Summary

Adds new style configuration options to allow users to customize the appearance of menus, command logs, and UI elements, providing finer control similar to Magit's transient face system.

New Style Options

Menu styles (style.menu.*):

• heading - Menu titles and section headings (e.g., "Commit", "Arguments")
• key - Keybinding display (e.g., "-a", "c", "q/esc")
• active_arg - Active argument values (e.g., "--all", "--interactive")
• inactive_arg - Inactive argument values

Message styles:

• info_msg - Info messages and command prompts (e.g., "$ ", "Running: ")
• error_msg - Error messages
• command - Command text in logs (e.g., "git status")

UI element styles:

• separator - Section separator lines
• prompt - Prompt text and separators

Technical Changes

• Fixed layout_line in src/ui.rs to properly merge line.style with span.style, ensuring Line::styled works correctly throughout the application
• Updated all configuration structs to support the new style options
• Updated 132 snapshot tests - all changes are only to styles_hash values which reflect the new style configurations

Snapshot Test Changes

All 132 snapshot test updates only change the styles_hash line (from old hash → new hash), confirming that:

• No visual output has unexpectedly changed
• Only the style configuration hash has been updated to reflect the new style options
• All rendering behavior remains functionally identical

Test Plan

• All existing tests pass (297 tests)
• Menu styles apply correctly to headings, keys, and arguments
• Message styles apply correctly to info/error messages and commands
• UI element styles apply correctly to separators and prompts
• Manual testing with Tokyo Night theme
• Verified snapshot changes are hash-only updates

🤖 Generated with Claude Code

[altsem]

Looks pretty good!