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/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)