app: unify 'not initialized' / 'not deployed' error phrasing across commands

[crowdy]

Background

PR #103 added mode-aware dispatch to nine `app` commands. Each site emits its own error for the "no marker" and "no CURRENT_SLOT" states, using different phrasing:

Site	Message
`deploy.go:60` (no-proxy)	`app %q not initialized on this server — run 'conoha app init --no-proxy --app-name %s %s' first`
`deploy.go:171` (proxy)	`app %q not initialized on this server — run 'conoha app init %s' first`
`rollback.go:56`	`app %q not initialized on this server — run 'conoha app init' first`
`logs/stop/restart/status`	`app %q has not been initialized on this server` (no recovery hint)
`logs/stop/restart` (slot check)	`app %q has not been deployed on this server`
`formatModeConflictError` (mode.go:73)	`... # re-initialize in %s mode` — embeds a literal `` token rather than the real server ID

Proposal

Add two helpers in `cmd/app/mode.go`:

```go
// notInitializedError returns the canonical error for "no .conoha-mode marker".
// If mode is non-empty, the recovery hint includes the right --no-proxy flag.
func notInitializedError(app, serverID string, mode Mode) error

// notDeployedError returns the canonical error for "marker present but
// CURRENT_SLOT absent" (= app was never deployed).
func notDeployedError(app string) error
```

Also thread `serverID` through `formatModeConflictError` so the printed recovery commands reference the actual server.

Acceptance

• All "not initialized" / "not deployed" errors across the `app` subcommands go through one helper each.
• `formatModeConflictError` prints the real server ID instead of ``.
• Tests updated to assert the new helper output.

Pointer

Surfaced by review on PR #103 (items M1, M2).