feat: Stage 5 — user management and API key lifecycle

Add full user management section (tab to Users):
- User list table with username, email, created date
- n: create user (username + email form)
- d: delete user with confirmation (cascades API keys)
- k: API key management for selected user
  - n: create new key with name input; one-time reveal
    showing key value and integer key ID prominently
  - r: revoke by integer ID (no list endpoint on server)
- c: copy revealed key to clipboard (atotto/clipboard)

API key listing is unavailable server-side, so the reveal
screen displays the key ID prominently for future revocation.

go.mod

@@ -2,12 +2,17 @@ module github.com/yeniklas/terdut-tui
 
 go 1.25.9
 
+require (
+	github.com/atotto/clipboard v0.1.4
+	github.com/charmbracelet/bubbles v1.0.0
+	github.com/charmbracelet/bubbletea v1.3.10
+	github.com/charmbracelet/lipgloss v1.1.0
+	gopkg.in/yaml.v3 v3.0.1
+)
+
 require (
 	github.com/aymanbagabas/go-osc52/v2 v2.0.1 // indirect
-	github.com/charmbracelet/bubbles v1.0.0 // indirect
-	github.com/charmbracelet/bubbletea v1.3.10 // indirect
 	github.com/charmbracelet/colorprofile v0.4.1 // indirect
-	github.com/charmbracelet/lipgloss v1.1.0 // indirect
 	github.com/charmbracelet/x/ansi v0.11.6 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect
 	github.com/charmbracelet/x/term v0.2.2 // indirect
@@ -26,5 +31,4 @@ require (
 	github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e // indirect
 	golang.org/x/sys v0.38.0 // indirect
 	golang.org/x/text v0.3.8 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
 )