feat(vault-tui): implement KV client and service for managing secrets

- Added internal/vault/client.go for creating a Vault client with configuration settings.
- Introduced internal/vault/errors.go to classify Vault API errors for better UI handling.
- Created internal/vault/kv.go to manage KV secrets, including listing, reading, writing, and deleting operations.
- Implemented internal/vault/mounts.go to list and describe secret engine mounts.
- Developed internal/vault/service.go to provide a unified entry point for Vault operations.
- Added internal/vault/kv_test.go for comprehensive testing of KV operations.
- Introduced internal/ui/toast.go for transient notifications in the UI.
- Added renovate.json for dependency management and updates.

internal/ui/overlay.go

@@ -0,0 +1,43 @@
+package ui
+
+import "charm.land/lipgloss/v2"
+
+// overlay composites box on top of base, centered over the current
+// terminal size, using lipgloss/v2's real layer compositor. This is what
+// makes confirm dialogs, the help screen, and the profile picker float
+// over the current screen instead of being appended below it.
+func (m *Model) overlay(base, box string) string {
+	bw, bh := lipgloss.Size(box)
+	x := (m.lay.w - bw) / 2
+	y := (m.lay.h - bh) / 2
+	if x < 0 {
+		x = 0
+	}
+	if y < 0 {
+		y = 0
+	}
+	return lipgloss.NewCompositor(
+		lipgloss.NewLayer(base),
+		lipgloss.NewLayer(box).X(x).Y(y).Z(1),
+	).Render()
+}
+
+const (
+	modalMaxContentWidth = 64
+	modalFrameWidth      = 10 // border(2) + padding(4, from Padding(1,2)) + a little breathing room
+	modalMinContentWidth = 24
+)
+
+// modalWrapWidth is the text-wrap width a screen should use before handing
+// content to styles.Modal/ModalDanger — keeps the box from ever exceeding
+// the terminal or the design's max modal width.
+func (m *Model) modalWrapWidth() int {
+	w := m.lay.w - modalFrameWidth
+	if w > modalMaxContentWidth {
+		w = modalMaxContentWidth
+	}
+	if w < modalMinContentWidth {
+		w = modalMinContentWidth
+	}
+	return w
+}