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/msgs.go

@@ -0,0 +1,90 @@
+package ui
+
+import (
+	"time"
+
+	"git.morlana.online/f.weber/vault-tui/internal/auth"
+	"git.morlana.online/f.weber/vault-tui/internal/token"
+	"git.morlana.online/f.weber/vault-tui/internal/vault"
+)
+
+// Every Vault/auth call the TUI makes is issued as a tea.Cmd and reported
+// back as one of these typed messages — Update never blocks on I/O itself.
+
+type mountsMsg struct {
+	mounts []vault.Mount
+	err    error
+}
+
+type listMsg struct {
+	listing vault.Listing
+	mount   vault.Mount
+	path    string
+	err     error
+}
+
+type secretMsg struct {
+	secret *vault.Secret
+	mount  vault.Mount
+	path   string
+	err    error
+}
+
+type versionsMsg struct {
+	versions []vault.VersionMeta
+	err      error
+}
+
+type writeAckMsg struct {
+	ack *vault.WriteAck
+	err error
+}
+
+type deleteAckMsg struct {
+	ack *vault.DeleteAck
+	err error
+}
+
+type rollbackAckMsg struct {
+	ack *vault.WriteAck
+	err error
+}
+
+type undeleteAckMsg struct {
+	ack *vault.DeleteAck
+	err error
+}
+
+type profileSwitchMsg struct {
+	profile string
+	err     error
+}
+
+type logoutDoneMsg struct{ err error }
+
+// clipboardClearMsg fires Settings.ClipboardClear after a copy, so a
+// secret value doesn't linger in the system clipboard indefinitely.
+type clipboardClearMsg struct{ seq int }
+
+type tokenInfoMsg struct {
+	info *token.Info
+	err  error
+}
+
+// loginEventMsg/loginDoneMsg drive the OIDC/Okta progress pump: Login runs
+// in a goroutine and reports each auth.Event plus a final result, exactly
+// mirroring the channel-pump pattern used by internal/cli's drainEvents.
+type loginEventMsg auth.Event
+
+type loginDoneMsg struct {
+	result    *auth.Result
+	err       error
+	storeWarn string // non-empty if login succeeded but token persistence failed
+}
+
+type errMsg struct{ err error }
+
+type statusMsg struct{ text string }
+
+// tickMsg drives the token-TTL countdown in the status bar.
+type tickMsg time.Time