refactor: modularize nvim config into lua/config + lua/plugins

- Extract all options, keymaps, and autocommands into lua/config/
- Move every plugin spec into its own file under lua/plugins/
- Add custom inline orng colorscheme (dark + light) synced to macOS appearance
- Consolidate image.nvim into pdf-viewer.lua (fix duplicate spec)
- Fix image.nvim processor: magick -> magick_cli
- Remove conflicting nvim-autopairs (kept mini.pairs)
- Fix broken <C-f> keymap (was calling non-existent builtin cmd)
- Delete lua/kickstart/ and lua/custom/ folders
- Add colors/orng.vim + colors/orng-light.vim stubs for mid-session switching
- init.lua reduced from 1078 lines to 49 lines

lua/plugins/treesitter.lua

@@ -0,0 +1,24 @@
+-- lua/plugins/treesitter.lua
+-- Syntax highlighting, indentation, and more via Tree-sitter
+
+return {
+  'nvim-treesitter/nvim-treesitter',
+  build = ':TSUpdate',
+  main  = 'nvim-treesitter.configs',
+  opts = {
+    ensure_installed = {
+      'bash', 'c', 'diff', 'html', 'lua', 'luadoc',
+      'markdown', 'markdown_inline', 'query', 'vim', 'vimdoc',
+      -- web
+      'javascript', 'typescript', 'tsx', 'json', 'css',
+      -- go / php
+      'go', 'php',
+    },
+    auto_install = true,
+    highlight = {
+      enable = true,
+      additional_vim_regex_highlighting = { 'ruby' },
+    },
+    indent = { enable = true, disable = { 'ruby' } },
+  },
+}