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/snapshot/module.go

@@ -1,19 +1,18 @@
 package snapshot
 
 import (
-	"io"
-
 	"github.com/spf13/afero"
 	"go.uber.org/fx"
 	"sneak.berlin/go/vaultik/internal/config"
 	"sneak.berlin/go/vaultik/internal/database"
 	"sneak.berlin/go/vaultik/internal/storage"
+	"sneak.berlin/go/vaultik/internal/ui"
 )
 
 // ScannerParams holds parameters for scanner creation
 type ScannerParams struct {
 	EnableProgress bool
-	Output         io.Writer // Where one-off scanner messages go; nil disables them
+	UI             *ui.Writer // Where user-facing scanner messages go; nil = discard
 	Fs             afero.Fs
 	Exclude        []string // Exclude patterns (combined global + snapshot-specific)
 	SkipErrors     bool     // Skip file read errors (log loudly but continue)
@@ -49,7 +48,7 @@ func provideScannerFactory(cfg *config.Config, repos *database.Repositories, sto
 			CompressionLevel: cfg.CompressionLevel,
 			AgeRecipients:    cfg.AgeRecipients,
 			EnableProgress:   params.EnableProgress,
-			Output:           params.Output,
+			UI:               params.UI,
 			Exclude:          excludes,
 			SkipErrors:       params.SkipErrors,
 		})