fix(frontend): seed full Zod-schema defaults for stream slices + QUIC params (B17)

XHTTP showed blank Selects for Session Placement / Sequence Placement /
Padding Method / Uplink HTTP Method (and several other knobs). Those
fields have a literal "" (empty string) value in the schema, which the
Select renders as "Default (path)" / "Default (repeat-x)" / etc.
The form field was `undefined`, not `""`, so the Select showed blank
instead of the labelled default option.

newStreamSlice in InboundFormModal hand-rolled per-network seed
objects with only a handful of fields. Replaced with
{Tcp,Kcp,Ws,Grpc,HttpUpgrade,XHttp}StreamSettingsSchema.parse({}) so
every default declared in the schema populates the form on network
switch. Same change in buildAddModeValues for the initial TCP state.

QUIC Params (FinalMaskForm) had the same shape on a smaller scale —
defaultQuicParams() only seeded congestion + debug + udpHop. The
schema's other fields are .optional() (no Zod default) so a schema
parse won't help. Hard-coded the xray-core / hysteria recommended
values (maxIdleTimeout 30, keepAlivePeriod 10, brutalUp/Down 0,
maxIncomingStreams 1024, four window sizes) so the InputNumber
controls render with usable starting values instead of blank.

frontend/src/components/FinalMaskForm.tsx

@@ -80,10 +80,26 @@ function defaultNoiseItem(): Record<string, unknown> {
 }
 
 function defaultQuicParams(): Record<string, unknown> {
+  // Seeded with the xray-core / hysteria recommended defaults so the QUIC
+  // Params sub-form doesn't show blank InputNumber fields when first
+  // enabled. The schema declares these as .optional() (no Zod default)
+  // because the wire shape omits them when xray's built-in default
+  // applies — but the panel needs values to render the controls.
   return {
     congestion: 'bbr',
     debug: false,
+    brutalUp: 0,
+    brutalDown: 0,
+    hasUdpHop: false,
     udpHop: { ports: '20000-50000', interval: 5 },
+    maxIdleTimeout: 30,
+    keepAlivePeriod: 10,
+    disablePathMTUDiscovery: false,
+    maxIncomingStreams: 1024,
+    initStreamReceiveWindow: 8388608,
+    maxStreamReceiveWindow: 8388608,
+    initConnectionReceiveWindow: 20971520,
+    maxConnectionReceiveWindow: 20971520,
   };
 }