Settings: Save and Cancel buttons instead of saving each change

Changes made in the Settings window show at once but are only kept with
the Save button, placed under the tab labels; it takes the accent color
while changes are waiting. Cancel goes back to the saved settings, and
closing the window (or the page) with unsaved changes asks first.

Tags are now sent as a whole list (PUT /api/tags, nothing saved when one
is invalid, the error gives its position) and "Restore default tags"
shows GET /api/tags/defaults until Save.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tags.go

@@ -217,6 +217,51 @@ func (s *TagStore) Delete(id string) error {
 	return errTagNotFound
 }
 
+// tagListError tells which tag of a list (PUT /api/tags) is invalid.
+type tagListError struct {
+	index int
+	err   error
+}
+
+func (e *tagListError) Error() string { return e.err.Error() }
+func (e *tagListError) Unwrap() error { return e.err }
+
+// Replace saves a whole list at once (the Save button of the settings). A tag
+// whose id is known keeps its code; the others get a new id and a free code.
+// Nothing is saved when a tag is invalid: the error gives its position.
+func (s *TagStore) Replace(in []Tag) ([]Tag, error) {
+	if len(in) > 99 {
+		return nil, errTooManyTags
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	codes := map[string]string{}
+	for _, t := range s.tags {
+		codes[t.ID] = t.Code
+	}
+	out := make([]Tag, len(in))
+	seen := map[string]bool{}
+	for i, t := range in {
+		if err := t.validate(); err != nil {
+			return nil, &tagListError{index: i, err: err}
+		}
+		if c, ok := codes[t.ID]; ok && !seen[t.ID] {
+			t.Code = c
+		} else {
+			t.ID, t.Code = newID(), ""
+		}
+		seen[t.ID] = true
+		out[i] = t
+	}
+	for i := range out {
+		if out[i].Code == "" {
+			out[i].Code = freeCode(out)
+		}
+	}
+	s.tags = out
+	return append([]Tag(nil), out...), s.save()
+}
+
 func (s *TagStore) Reset() ([]Tag, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()