feat(nvim): toggle terminal as a split via <leader>t{h,v}

replace the floating `Ctrl-\` toggle (unergonomic on a German layout) with
`<leader>th`/`<leader>tv` for horizontal/vertical splits; `<C-w>` in the
terminal leaves to normal mode in one chord.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nvim.nix

@@ -199,13 +199,14 @@ in
         # consumer wanting auto-restore on a bare `nvim` adds a VimEnter autocmd.
         persistence.enable = true;
 
-        # Floating terminal toggled with Ctrl-\ (shares nvim's cwd). Try it; if
+        # Terminal split (shares nvim's cwd), toggled via <leader>t{h,v} below.
-        # a tiled foot next to nvim serves you better, drop this block.
+        # No open_mapping — the leader binds cover both directions; size applies
+        # to whichever split is open (rows when horizontal, cols when vertical).
         toggleterm = {
           enable = true;
           settings = {
-            open_mapping = { __raw = "[[<c-\\>]]"; };
+            direction = "horizontal";
-            direction = "float";
+            size = 15;
           };
         };
 
@@ -221,6 +222,7 @@ in
             { __unkeyed-1 = "<leader>c"; group = "Code"; }
             { __unkeyed-1 = "<leader>r"; group = "Refactor"; }
             { __unkeyed-1 = "<leader>s"; group = "Search"; }
+            { __unkeyed-1 = "<leader>t"; group = "Terminal"; }
           ];
         };
       };
@@ -251,6 +253,13 @@ in
         { mode = "n"; key = "<S-h>"; action = "<cmd>bprevious<cr>"; options.desc = "Prev buffer"; }
         { mode = "n"; key = "<leader>bd"; action = "<cmd>bdelete<cr>"; options.desc = "Close buffer"; }
 
+        # Terminal: toggle a horizontal or vertical split (toggleterm). Same key
+        # hides it again. From inside the terminal, <C-w> drops to normal mode and
+        # runs the window command, so leaving is one chord (no <C-\><C-n> dance).
+        { mode = "n"; key = "<leader>th"; action = "<cmd>ToggleTerm direction=horizontal<cr>"; options.desc = "Terminal (horizontal)"; }
+        { mode = "n"; key = "<leader>tv"; action = "<cmd>ToggleTerm direction=vertical size=80<cr>"; options.desc = "Terminal (vertical)"; }
+        { mode = "t"; key = "<C-w>"; action = ''<C-\><C-n><C-w>''; options.desc = "Leave terminal to window"; }
+
         # Cheatsheet: <leader>? pops the full which-key list; <leader>sk fuzzy-
         # searches every active mapping via Telescope (catches non-leader binds too).
         { mode = "n"; key = "<leader>?"; action = "<cmd>WhichKey<cr>"; options.desc = "All keybinds (which-key)"; }