Introduce internal/ui package and rewrite user-facing output

All user-facing output now goes through a single ui.Writer with a
uniform style:

  》 (white)     for begin / info / notice
  》 (green)     for complete / success
  Warning:      for warnings (orange)
  ERROR:        for errors (red)
    》          (indented) for progress heartbeats

Color is enabled when stdout is a TTY and NO_COLOR is unset.

Standards:
- Complete-sentence messages with fully qualified terms ("backup
  destination store", "local index database", "snapshot source
  files enumeration").
- Every Complete has a matching Begin.
- Natural verb tense conveys state ("Uploading" -> "Uploaded"). The
  words "begin"/"complete" never appear in message bodies; the marker
  color carries that information.
- ETA means clock time, not duration. Progress lines say "estimated
  remaining time (<dur>), finish at <time>" with both labeled.

Adds globals.CommitDate (populated by Makefile/Dockerfile/goreleaser
via ldflags from `git show -s --format=%cI HEAD`) and a startup banner
printed once per invocation.

Strips fx call-chain noise from startup errors so users see the actual
underlying error (e.g. "creating base path: mkdir /Volumes/BACKUPS:
permission denied" instead of three layers of "could not build
arguments for function ...").

README documents the output style and the ui package conventions.

internal/cli/app.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -20,6 +21,7 @@ import (
 	"sneak.berlin/go/vaultik/internal/pidlock"
 	"sneak.berlin/go/vaultik/internal/snapshot"
 	"sneak.berlin/go/vaultik/internal/storage"
+	"sneak.berlin/go/vaultik/internal/ui"
 	"sneak.berlin/go/vaultik/internal/vaultik"
 )
 
@@ -33,11 +35,21 @@ type AppOptions struct {
 	Invokes    []fx.Option
 }
 
-// setupGlobals sets up the globals with application startup time
-func setupGlobals(lc fx.Lifecycle, g *globals.Globals) {
+// setupGlobals records the startup time and prints the startup banner.
+// In --cron mode the banner is suppressed (LogOptions.Cron == true).
+func setupGlobals(lc fx.Lifecycle, g *globals.Globals, v *vaultik.Vaultik, opts log.LogOptions) {
 	lc.Append(fx.Hook{
 		OnStart: func(ctx context.Context) error {
 			g.StartTime = time.Now().UTC()
+			if opts.Cron || opts.Quiet {
+				// Replace UI writer with a discarding one so all
+				// user-facing output is suppressed.
+				v.UI = ui.NewWithColor(io.Discard, false)
+			} else {
+				v.UI.Banner("%s %s (commit %s, %s) invoked at %s by %s",
+					g.Appname, g.Version, g.ShortCommit(), g.CommitDate,
+					g.StartTime.Format(time.RFC3339), globals.Author)
+			}
 			return nil
 		},
 	})

internal/cli/snapshot.go

@@ -3,7 +3,6 @@ package cli
 import (
 	"context"
 	"fmt"
-	"io"
 	"os"
 
 	"github.com/spf13/cobra"
@@ -73,9 +72,7 @@ specifying a path using --config or by setting VAULTIK_CONFIG to a path.`,
 							OnStart: func(ctx context.Context) error {
 								// Start the snapshot creation in a goroutine
 								go func() {
-									if opts.Cron {
-										v.Stdout = io.Discard
-									}
+									// --cron suppression is wired through v.UI by setupGlobals.
 									if err := v.CreateSnapshot(opts); err != nil {
 										if err != context.Canceled {
 											log.Error("Snapshot creation failed", "error", err)