nixvim: improve keymaps, completion, and formatter config

- Add LSP code actions, diagnostic navigation, window navigation keymaps
- Move telescope keymaps to explicit mkRaw form for reliability
- Change buffer nav from <leader>bh/bl to [b/]b
- Fix conform double-setup by moving config to native nixvim settings
- Fix <leader>cf to use conform instead of vim.lsp.buf.format
- Fix <leader>ca to pass diagnostic context to code_action
- Add nvim-lightbulb for code action indicator
- Configure copilot Tab conflict: blink-cmp Tab=accept, C-n/p for cycling
- Add completion.list.selection preselect for immediate Tab acceptance
- Add TypeScript Japanese locale for LSP diagnostics

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

modules/home/nixvim/plugins/ui/lightbulb.nix

@@ -0,0 +1,6 @@
+{...}: {
+  programs.nixvim.plugins.nvim-lightbulb = {
+    enable = true;
+    settings.autocmd.enabled = true;
+  };
+}