feat(plugin): modernize plugin configurations and add colorscheme

- Fix plugin quoting and require calls in 00‑colorscheme.lua
- Set gruvbox colorscheme with transparent mode and updated overrides  - Update snacks.nvim layout to use string quotes and minor option tweaks
- Add markdown‑preview plugin with build step
- Consolidate configuration keys and remove deprecated settings

plugin/00-colorscheme.lua

@@ -1,34 +1,36 @@
-add("ellisonleao/gruvbox.nvim")
+add('ellisonleao/gruvbox.nvim')
 
-local palette = require("gruvbox").palette
-require("gruvbox").setup({
-	bold = false,
-	italic = {
-		strings = false,
-		emphasis = false,
-		comments = false,
-		folds = false,
-	},
-	contrast = "hard",
-	transparent_mode = true,
-	overrides = {
-		-- builtin
-		SignColumn = { bg = "None" },
-		CursorLineNr = { bg = "None" },
-		Whitespace = { fg = palette.dark0 },
-		EndOfBuffer = { fg = palette.dark0_hard },
-		Visual = { fg = "None", bg = palette.dark1 },
-		Label = { link = "GruvboxPurple" },
+local palette = require('gruvbox').palette
+require('gruvbox').setup({
+    bold = false,
+    italic = {
+        strings = false,
+        emphasis = false,
+        comments = false,
+        folds = false,
+    },
+    contrast = 'hard',
+    transparent_mode = true,
+    overrides = {
+        -- builtin
+        SignColumn = { bg = 'None' },
+        CursorLineNr = { bg = 'None' },
+        Whitespace = { fg = palette.dark0 },
+        EndOfBuffer = { fg = palette.dark0_hard },
+        Visual = { fg = 'None', bg = palette.dark1 },
+        Label = { link = 'GruvboxPurple' },
 
-		-- blink.cmp
-		BlinkCmpMenu = { link = "Normal" },
-		BlinkCmpMenuBorder = { link = "BlinkCmpMenu" },
-		BlinkCmpMenuSelection = { fg = "None", bg = palette.dark1 },
-		BlinkCmpLabelMatch = { fg = palette.bright_green },
-		BlinkCmpScrollBarGutter = { bg = palette.dark2 },
-		BlinkCmpScrollBarThumb = { bg = palette.light1 },
+        -- blink.cmp
+        BlinkCmpMenu = { link = 'Normal' },
+        BlinkCmpMenuBorder = { link = 'BlinkCmpMenu' },
+        BlinkCmpMenuSelection = { fg = 'None', bg = palette.dark1 },
+        BlinkCmpLabelMatch = { fg = palette.bright_green },
+        BlinkCmpScrollBarGutter = { bg = palette.dark2 },
+        BlinkCmpScrollBarThumb = { bg = palette.light1 },
 
-		-- indent guides
-		MiniIndentscopeSymbol = { fg = palette.dark1 },
-	},
+        -- indent guides
+        MiniIndentscopeSymbol = { fg = palette.dark1 },
+    },
 })
+
+vim.cmd('colorscheme gruvbox')