refactor(xhttp): split fields by direction, expand outbound coverage

Audit panel xhttp config against xray-core's runtime paths and split
fields per direction so each side carries only what it actually uses:

- Bidirectional (must match): host, path, mode, all xPadding*,
  session*/seq*, uplinkData*/Key, scMaxEachPostBytes
- Server-only (inbound): noSSEHeader, scMaxBufferedPosts,
  scStreamUpServerSecs, serverMaxHeaderBytes
- Client-only (outbound): uplinkHTTPMethod, uplinkChunkSize,
  noGRPCHeader, scMinPostsIntervalMs, xmux

The inbound previously held client-only fields and the outbound was
missing every must-match field beyond host/path/mode — meaning a
panel-built outbound couldn't connect to an inbound with a custom
xPaddingKey/sessionKey/etc.

Headers stay on the inbound for URL-share purposes only; xray's
listener ignores them at runtime, but they travel through the share
link's `extra` blob so the client picks them up.

Renames the URL helpers (applyXhttpPadding* -> applyXhttpExtra*) since
the blob now carries more than padding, and folds path/host/mode into
the helper so each link generator's xhttp branch is one line.

Adds two enforcement points for xray's "uplinkHTTPMethod=GET only in
packet-up" rule: the GET option is disabled when mode != packet-up,
and a watcher on the outbound modal auto-clears GET when the user
switches modes.

Hides the XMUX block behind an `enableXmux` switch on the outbound
form (mirrors the QUIC Params toggle) so the section doesn't clutter
the form by default; fromJson auto-flips it on for outbounds with
saved xmux config.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/html/form/stream/stream_xhttp.html

@@ -37,6 +37,10 @@
   <a-form-item label="Stream-Up Server" v-if="inbound.stream.xhttp.mode === 'stream-up'">
     <a-input v-model.trim="inbound.stream.xhttp.scStreamUpServerSecs"></a-input>
   </a-form-item>
+  <a-form-item label="Server Max Header Bytes">
+    <a-input-number v-model.number="inbound.stream.xhttp.serverMaxHeaderBytes" :min="0"
+      placeholder="0 (default)"></a-input-number>
+  </a-form-item>
   <a-form-item label="Padding Bytes">
     <a-input v-model.trim="inbound.stream.xhttp.xPaddingBytes"></a-input>
   </a-form-item>
@@ -67,14 +71,6 @@
       </a-select>
     </a-form-item>
   </template>
-  <a-form-item label="Uplink HTTP Method">
-    <a-select v-model="inbound.stream.xhttp.uplinkHTTPMethod" :dropdown-class-name="themeSwitcher.currentTheme">
-      <a-select-option value>Default (POST)</a-select-option>
-      <a-select-option value="POST">POST</a-select-option>
-      <a-select-option value="PUT">PUT</a-select-option>
-      <a-select-option value="GET">GET (packet-up only)</a-select-option>
-    </a-select>
-  </a-form-item>
   <a-form-item label="Session Placement">
     <a-select v-model="inbound.stream.xhttp.sessionPlacement" :dropdown-class-name="themeSwitcher.currentTheme">
       <a-select-option value>Default (path)</a-select-option>
@@ -114,13 +110,8 @@
     v-if="inbound.stream.xhttp.mode === 'packet-up' && inbound.stream.xhttp.uplinkDataPlacement && inbound.stream.xhttp.uplinkDataPlacement !== 'body'">
     <a-input v-model.trim="inbound.stream.xhttp.uplinkDataKey" placeholder="x_data"></a-input>
   </a-form-item>
-  <a-form-item label="Uplink Chunk Size"
-    v-if="inbound.stream.xhttp.mode === 'packet-up' && inbound.stream.xhttp.uplinkDataPlacement && inbound.stream.xhttp.uplinkDataPlacement !== 'body'">
-    <a-input-number v-model.number="inbound.stream.xhttp.uplinkChunkSize" :min="0"
-      placeholder="0 (unlimited)"></a-input-number>
-  </a-form-item>
   <a-form-item label="No SSE Header">
     <a-switch v-model="inbound.stream.xhttp.noSSEHeader"></a-switch>
   </a-form-item>
 </a-form>
-{{end}}
+{{end}}