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/globals/globals.go

@@ -13,19 +13,36 @@ var Version string = "dev"
 // Commit is the git commit hash, populated from main().
 var Commit string = "unknown"
 
+// CommitDate is the ISO-8601 date of the commit, populated from main().
+var CommitDate string = "unknown"
+
+// Author identifies the upstream author of vaultik.
+const Author = "Jeffrey Paul <sneak@sneak.berlin>"
+
 // Globals contains application-wide configuration and metadata.
 type Globals struct {
-	Appname   string
-	Version   string
-	Commit    string
-	StartTime time.Time
+	Appname    string
+	Version    string
+	Commit     string
+	CommitDate string
+	StartTime  time.Time
 }
 
 // New creates and returns a new Globals instance initialized with the package-level variables.
 func New() (*Globals, error) {
 	return &Globals{
-		Appname: Appname,
-		Version: Version,
-		Commit:  Commit,
+		Appname:    Appname,
+		Version:    Version,
+		Commit:     Commit,
+		CommitDate: CommitDate,
 	}, nil
 }
+
+// ShortCommit returns the first 12 chars of the commit hash, or the
+// whole string if it's shorter (e.g. "unknown").
+func (g *Globals) ShortCommit() string {
+	if len(g.Commit) > 12 {
+		return g.Commit[:12]
+	}
+	return g.Commit
+}