refactor(presets): replace xdg-open with vim.ui.open

Problem: all presets hardcoded `open = { 'xdg-open' }`, making them
Linux-only. The compiler already handles `open = true` via
`vim.ui.open()`, which is cross-platform.

Solution: change all four presets to `open = true`.

lua/preview/presets.lua

@@ -10,7 +10,7 @@ M.typst = {
   output = function(ctx)
     return (ctx.file:gsub('%.typ$', '.pdf'))
   end,
-  open = { 'xdg-open' },
+  open = true,
 }
 
 ---@type preview.ProviderConfig
@@ -26,7 +26,7 @@ M.latex = {
   clean = function(ctx)
     return { 'latexmk', '-c', ctx.file }
   end,
-  open = { 'xdg-open' },
+  open = true,
 }
 
 ---@type preview.ProviderConfig
@@ -43,7 +43,7 @@ M.markdown = {
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
   end,
-  open = { 'xdg-open' },
+  open = true,
 }
 
 ---@type preview.ProviderConfig
@@ -70,7 +70,7 @@ M.github = {
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
   end,
-  open = { 'xdg-open' },
+  open = true,
 }
 
 return M

spec/presets_spec.lua

@@ -34,7 +34,7 @@ describe('presets', function()
     end)
 
     it('has open enabled', function()
-      assert.are.same({ 'xdg-open' }, presets.typst.open)
+      assert.is_true(presets.typst.open)
     end)
   end)
 
@@ -73,7 +73,7 @@ describe('presets', function()
     end)
 
     it('has open enabled', function()
-      assert.are.same({ 'xdg-open' }, presets.latex.open)
+      assert.is_true(presets.latex.open)
     end)
   end)
 
@@ -115,7 +115,7 @@ describe('presets', function()
     end)
 
     it('has open enabled', function()
-      assert.are.same({ 'xdg-open' }, presets.markdown.open)
+      assert.is_true(presets.markdown.open)
     end)
   end)
 
@@ -177,7 +177,7 @@ describe('presets', function()
     end)
 
     it('has open enabled', function()
-      assert.are.same({ 'xdg-open' }, presets.github.open)
+      assert.is_true(presets.github.open)
     end)
   end)
 end)