feat(nvim): accept completion with <CR>, indent 4

blink.cmp uses the `enter` preset with `completion.list.selection.preselect =
false`, so an item is taken only once selected with `<Down>`/`<C-n>` and `<CR>`
otherwise inserts a newline. `<C-y>` stays bound as vim's own confirm key.

`<Esc>` and `<C-e>` both run `cancel` rather than the preset's `hide`: hiding
leaves the auto_insert preview in the buffer without the `additionalTextEdits`
and snippet expansion a real accept applies, and builtin ins-completion `<C-e>`
likewise restores the typed text.

`shiftwidth`/`tabstop` are 4, matching PHP's PSR-12. PHP's `indentkeys` carry
`*<Return>`, which reindents the just-finished line on every newline; at width 2
that silently re-indented 4-space files.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015D4NsszUG7hy9ghKZ4nfhm

modules/nvim.nix

@@ -86,8 +86,8 @@ in
         scrolloff = 8;              # keep 8 lines of context around the cursor
 
         expandtab = true;           # spaces, not tabs
-        shiftwidth = 2;
+        shiftwidth = 4;
-        tabstop = 2;
+        tabstop = 4;
         smartindent = true;
 
         ignorecase = true;          # case-insensitive search…
@@ -162,10 +162,31 @@ in
         blink-cmp = {
           enable = true;
           settings = {
-            keymap.preset = "default";   # C-space open, C-y confirm, C-n/C-p move
+            # "enter" preset: <CR> accepts, Up/Down and C-n/C-p move, Tab is
+            # left to snippet jumping. Custom keys below merge over the preset.
+            keymap = {
+              preset = "enter";
+              # Vim's own ins-completion confirm key, kept alongside <CR>.
+              "<C-y>" = [ "select_and_accept" "fallback" ];
+              # Dismiss the menu and undo the auto_insert preview, staying in
+              # insert mode. `cancel` rather than the preset's `hide`, which
+              # would leave the previewed text in the buffer without the
+              # additionalTextEdits (imports) and snippet expansion that a real
+              # accept applies — a degraded accept nobody wants. This also
+              # matches builtin ins-completion, where C-e restores the typed
+              # text. With no menu open both keys fall back to their native
+              # meaning (Esc leaves insert mode).
+              "<C-e>" = [ "cancel" "fallback" ];
+              "<Esc>" = [ "cancel" "fallback" ];
+            };
             appearance.nerd_font_variant = "mono";
             sources.default = [ "lsp" "path" "snippets" "buffer" ];
-            completion.documentation.auto_show = true;
+            completion = {
+              documentation.auto_show = true;
+              # No item is selected until one is picked with Down/C-n, so <CR>
+              # inserts a newline unless a completion was deliberately chosen.
+              list.selection.preselect = false;
+            };
           };
         };