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/vaultik/vaultik.go

@@ -15,6 +15,7 @@ import (
 	"sneak.berlin/go/vaultik/internal/globals"
 	"sneak.berlin/go/vaultik/internal/snapshot"
 	"sneak.berlin/go/vaultik/internal/storage"
+	"sneak.berlin/go/vaultik/internal/ui"
 )
 
 // Vaultik contains all dependencies needed for vaultik operations
@@ -37,6 +38,12 @@ type Vaultik struct {
 	Stdout io.Writer
 	Stderr io.Writer
 	Stdin  io.Reader
+
+	// UI is the writer for user-facing status, progress, warnings, errors.
+	// See package internal/ui for formatting conventions. Defaults to a
+	// writer wrapping Stdout; the cli layer replaces it with a discarding
+	// writer in --cron mode.
+	UI *ui.Writer
 }
 
 // VaultikParams contains all parameters for New that can be provided by fx
@@ -83,6 +90,7 @@ func New(params VaultikParams) *Vaultik {
 		Stdout:          os.Stdout,
 		Stderr:          os.Stderr,
 		Stdin:           os.Stdin,
+		UI:              ui.New(os.Stdout),
 	}
 }