feat: make mode colors user-configurable via g:tabs_vim_colors

[jesse23]

Summary

• Replace 14 hardcoded Dracula hi statements with a s:ApplyColors() function driven by a defaults dict
• Users can override any subset of mode colors via g:tabs_vim_colors in their vimrc; unspecified modes fall back to Dracula defaults
• Add ADR-002 documenting the theme contract and alternatives considered; update spec with the configuration contract and partial override example

Contract

" Each entry: [guifg, guibg, ctermfg, ctermbg] — any subset may be specified
let g:tabs_vim_colors = {
  \ 'normal': ['#282828', '#d79921', 235, 172],
  \ 'insert': ['#282828', '#b8bb26', 235, 142],
\ }

Test plan

• Load plugin with no g:tabs_vim_colors set — Dracula defaults apply unchanged
• Set a partial override (e.g. only normal) — overridden mode changes, others keep defaults
• Set a full override (all 6 modes) — all highlight groups reflect custom colors

[Copilot]

Pull request overview

Adds a user-facing theming contract so mode “pill” highlight colors can be configured via g:tabs_vim_colors, while keeping Dracula as the default palette.

Changes:

• Replaces hardcoded hi statements with an s:ApplyColors() function driven by a defaults dict and optional user overrides.
• Documents the configuration contract and partial override example in the spec.
• Adds ADR-002 capturing the theming decision and alternatives considered.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
plugin/tabs.vim	Introduces defaults + s:ApplyColors() to generate highlight groups from merged config.
docs/specs/tabs.vim.md	Specifies g:tabs_vim_colors contract, defaults, and partial override example.
docs/adrs/002.tabs-vim.themes.md	ADR documenting the chosen theming configuration approach and tradeoffs.

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