feat: add nvim-web-devicons support for note sign icon

[ushmz]

Summary

• Add automatic detection of nvim-web-devicons plugin to display appropriate icons in the sign column
• When nvim-web-devicons is available, use the comment icon
• When not available, fall back to a middle dot (･)

This enhances the visual experience for users with Nerd Fonts while maintaining compatibility for those without.

Changes

• Modified lua/tabi/ui/display.lua:init() to check for nvim-web-devicons availability using pcall
• Set sign text dynamically based on plugin availability

Test plan

• All existing tests pass (261/261)
• luacheck passes with no warnings/errors
• stylua formatting check passes
• Pre-commit hooks pass

TODO

• Add support for mini.icons as an alternative icon provider