feat(tui): add bubbletea terminal UI with entity list, detail, and capture

Adds `nib tui` command and `make tui` target. Scrollable entity list
with j/k navigation, enter for detail view, `a` to capture new entries
using the existing parse grammar, and `d` to delete.

Makefile

@@ -2,7 +2,7 @@ BINARY  := nib
 MODULE  := github.com/lerko/nib
 GOFLAGS := -trimpath
 
-.PHONY: build dev watch test lint fmt vet clean run cert help
+.PHONY: build dev tui watch test lint fmt vet clean run cert help
 
 ## —— Build ——————————————————————————————————
 
@@ -12,6 +12,9 @@ build: ## Build production binary
 dev: ## Build and run with default serve
 	go run . serve
 
+tui: ## Launch the terminal UI
+	go run . tui
+
 watch: ## Live-reload dev server (requires air)
 	air