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/modals/xray_outbound_modal.html

@@ -104,6 +104,17 @@
                 return outModal.outbound;
             },
         },
+        watch: {
+            // xray-core's SplitHTTPConfig.Build() rejects "GET" as
+            // uplinkHTTPMethod outside packet-up mode. Clear the field
+            // instead of carrying an invalid combination through.
+            "outModal.outbound.stream.xhttp.mode"(newMode) {
+                const xhttp = outModal.outbound.stream && outModal.outbound.stream.xhttp;
+                if (xhttp && xhttp.uplinkHTTPMethod === "GET" && newMode !== "packet-up") {
+                    xhttp.uplinkHTTPMethod = "";
+                }
+            },
+        },
         methods: {
             streamNetworkChange() {
                 if (this.outModal.outbound.protocol == Protocols.VLESS && !outModal.outbound