Keep the whole startup error when it contains "): " (closes #320)

A startup error was cut at its last "): " to drop the annotations fx
adds, so the blob_size_limit message, which itself contains
"(... size spread): ", lost everything but "need at least N bytes".
cleanStartupError now uses dig.RootCause, from the library fx is built
on, which removes exactly those annotations and returns the error
underneath unchanged.

The test starts a real fx app on a config whose blob_size_limit is below
the minimum. The old cases fed it plain strings that only looked like fx
output; such an error is now rightly left whole, so they are replaced.

go.uber.org/dig, already required through fx, becomes a direct
requirement in go.mod.

Model: opus-5-5

internal/cli/app.go

@@ -8,12 +8,12 @@ import (
 	"errors"
 	"fmt"
 	"path/filepath"
-	"strings"
 	"sync"
 	"time"
 
 	"github.com/adrg/xdg"
 	"github.com/spf13/cobra"
+	"go.uber.org/dig"
 	"go.uber.org/fx"
 	"sneak.berlin/go/vaultik/internal/config"
 	"sneak.berlin/go/vaultik/internal/database"
@@ -146,16 +146,12 @@ func (e *startupError) Error() string {
 //	could not build arguments for function "Y" (file:line): failed to build U:
 //	received non-nil error from function "Z" (file:line): <real error>
 //
-// Users care about the real error, not the DI plumbing. We strip everything
-// up through the last "): " (which is always the close-paren of an fx
-// function-location annotation followed by the wrapped error).
+// Users care about the real error, not the DI plumbing. dig.RootCause,
+// from the library fx is built on, unwraps those layers and returns the
+// real error whole. Cutting the text at "): " instead would also cut a
+// real error that contains it, such as the blob_size_limit one.
 func cleanStartupError(err error) error {
-	msg := err.Error()
-	if idx := strings.LastIndex(msg, "): "); idx >= 0 {
-		msg = msg[idx+3:]
-	}
-
-	return &startupError{msg: msg}
+	return &startupError{msg: dig.RootCause(err).Error()}
 }
 
 // RunApp starts the fx application, blocks until it is asked to stop, and