fix: client perms, @everyone/@channel, docs, unit tests

- usePermissions ORs current user roles + @everyone only (not all server roles)
- cache myRolesByServer; load on active server; refresh after self role edit
- gate/notify @everyone and @channel; plain @username push; special mention UI
- refresh FEATURE_PARITY (DMs exist; drop stale critical gaps)
- README production deploy notes dumpster.service
- unit tests for permission bits and broadcast mention tokens

internal/message/mentions_test.go

@@ -0,0 +1,38 @@
+package message
+
+import "testing"
+
+func TestHasBroadcastToken(t *testing.T) {
+	cases := []struct {
+		content string
+		token   string
+		want    bool
+	}{
+		{"hello @everyone", "everyone", true},
+		{"@everyone hi", "everyone", true},
+		{"@EVERYONE", "everyone", true},
+		{"@everyone!", "everyone", true},
+		{"@everyoneelse", "everyone", false},
+		{"noteveryone", "everyone", false},
+		{"@channel", "channel", true},
+		{"ping @channel please", "channel", true},
+		{"@channeling", "channel", false},
+		{"", "everyone", false},
+		{"@", "everyone", false},
+	}
+	for _, tc := range cases {
+		got := hasBroadcastToken(tc.content, tc.token)
+		if got != tc.want {
+			t.Errorf("hasBroadcastToken(%q, %q) = %v, want %v", tc.content, tc.token, got, tc.want)
+		}
+	}
+}
+
+func TestIsUsernameChar(t *testing.T) {
+	if !isUsernameChar('a') || !isUsernameChar('9') || !isUsernameChar('_') {
+		t.Fatal("expected alnum/_")
+	}
+	if isUsernameChar(' ') || isUsernameChar('!') || isUsernameChar('@') {
+		t.Fatal("unexpected username chars")
+	}
+}