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.

Makefile

@@ -5,10 +5,12 @@ VERSION := 1.0.0-rc.1
 
 # Build variables
 GIT_REVISION := $(shell git rev-parse HEAD 2>/dev/null || echo "unknown")
+GIT_COMMIT_DATE := $(shell git show -s --format=%cI HEAD 2>/dev/null || echo "unknown")
 
 # Linker flags
 LDFLAGS := -X 'sneak.berlin/go/vaultik/internal/globals.Version=$(VERSION)' \
-           -X 'sneak.berlin/go/vaultik/internal/globals.Commit=$(GIT_REVISION)'
+           -X 'sneak.berlin/go/vaultik/internal/globals.Commit=$(GIT_REVISION)' \
+           -X 'sneak.berlin/go/vaultik/internal/globals.CommitDate=$(GIT_COMMIT_DATE)'
 
 # Default target
 all: vaultik