fix(theme): support multi-codepoint emojis in theme.yml glyph field

[dcadenas]

Summary

Fixes #1654 - Adds support for multi-codepoint emojis in theme.yml glyph fields.

Problem

Emojis with Unicode variation selectors (🖼️, 🖥️) and other multi-codepoint sequences caused the entire theme.yml file to be silently ignored. The glyph field was typed as Option<char>, which can only hold single Unicode scalar values.

Solution

Changed the glyph field from Option<char> to Option<String> to support:

• ✅ Emojis with variation selectors (U+FE0F): 🖼️ 🖥️ ⌚️ ☎️
• ✅ ZWJ (Zero Width Joiner) sequences: 👨‍💻 👩‍🎨 👨‍👩‍👧‍👦
• ✅ Flag emojis (regional indicators): 🇺🇸 🇬🇧 🇯🇵
• ✅ Skin tone modifiers: 👋🏻 👋🏿

Also improved error handling to report theme parsing failures instead of silently failing.

Changes

Type Changes

• src/theme/ui_styles.rs: Changed IconStyle::glyph from Option<char> to Option<String>
• src/options/config.rs: Changed IconStyleOverride::glyph from Option<char> to Option<String>
• Removed Copy trait from structs containing String fields (IconStyle, FileNameStyle, IconStyleOverride, FileNameStyleOverride)

Function Updates

• src/output/icons.rs: Changed icon_for_file() return type from char to String
• src/output/file_name.rs: Updated icon usage to work with String type
• src/theme/mod.rs: Changed from dereferencing to cloning for non-Copy types

Error Handling

• src/options/config.rs: Replaced silent .ok() with explicit error messages when theme parsing fails

Testing

Added 7 comprehensive unit tests covering:

• Single-codepoint emojis (💾)
• Emojis with variation selectors (🖼️, 🖥️)
• ZWJ sequences (👨‍💻)
• Regional indicator sequences (🇺🇸)
• Mixed emoji themes
• Empty glyph strings
• Invalid YAML error handling

Test Results:

• All 462 tests pass ✅
• No clippy warnings ✅
• Code formatted with cargo fmt ✅

Backward Compatibility

Fully backward compatible - existing single-character icons continue to work without changes.

Example Usage

# ~/.config/eza/theme.yml
filenames:
  data: {icon: {glyph: 💾}}         # Single codepoint - works
  Pictures: {icon: {glyph: 🖼️}}     # Variation selector - now works!
  Desktop: {icon: {glyph: 🖥️}}      # Variation selector - now works!
  developer: {icon: {glyph: 👨‍💻}}   # ZWJ sequence - now works!

extensions:
  rs: {icon: {glyph: 🦀}}
  py: {icon: {glyph: 🐍}}

Checklist

• [x] Tests pass (cargo test)
• [x] No clippy warnings (cargo clippy)
• [x] Code formatted (cargo fmt)
• [x] Follows conventional commits
• [x] References issue (#1654)
• [x] Backward compatible