fix: rune-safe byte slicing in token estimator, error snippets, and maskKey

- restore/render.go: countTokens used len(text)/4 (bytes) instead of
  utf8.RuneCountInString(text)/4 (runes) — inflated estimate for any
  text containing multi-byte characters (CJK, emoji, accented chars)
- api/client.go: HTTP error snippet was truncated at byte offset 300,
  risking invalid UTF-8 when a multi-byte character straddles that boundary
- setup/wizard.go: maskKey sliced the key at byte positions 8 and len-4;
  same class of bug fixed elsewhere in the codebase

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: greynewell

internal/api/client.go

@@ -313,8 +313,8 @@ func (c *Client) request(ctx context.Context, method, path, contentType string,
 			return &apiErr
 		}
 		snippet := string(respBody)
-		if len(snippet) > 300 {
-			snippet = snippet[:300] + "..."
+		if runes := []rune(snippet); len(runes) > 300 {
+			snippet = string(runes[:300]) + "..."
 		}
 		return fmt.Errorf("HTTP %d: %s", resp.StatusCode, snippet)
 	}

internal/restore/render.go

@@ -7,6 +7,7 @@ import (
 	gotmpl "text/template"
 	"time"
 	"unicode"
+	"unicode/utf8"
 )
 
 const maxCyclesToShow = 10
@@ -288,7 +289,7 @@ func CountTokens(text string) int {
 			inWord = true
 		}
 	}
-	charEstimate := len(text) / 4
+	charEstimate := utf8.RuneCountInString(text) / 4
 	wordEstimate := words * 100 / 75
 	if charEstimate > wordEstimate {
 		return charEstimate

internal/setup/wizard.go

@@ -194,10 +194,11 @@ func boolPtr(b bool) *bool { return &b }
 
 // maskKey returns a display-safe version of the API key.
 func maskKey(key string) string {
-	if len(key) <= 12 {
-		return strings.Repeat("*", len(key))
+	runes := []rune(key)
+	if len(runes) <= 12 {
+		return strings.Repeat("*", len(runes))
 	}
-	return key[:8] + "..." + key[len(key)-4:]
+	return string(runes[:8]) + "..." + string(runes[len(runes)-4:])
 }
 
 // findGitRoot detects the git root from the current working directory.