Make SUB2API proxy defaults available across panel flows

Expose a configurable default proxy for SUB2API, resolve the proxy once,
persist the proxy_id in state, and reuse it across auth URL generation,
code exchange, and account creation so one OAuth flow stays on one proxy.

Constraint: Upstream GitHub permission is READ, so publication must use a fork PR
Rejected: Push to origin | current GitHub viewer permission does not allow upstream branch publication
Rejected: Resolve proxy independently in later steps | could drift from the proxy chosen at auth URL generation
Confidence: high
Scope-risk: moderate
Reversibility: clean
Tested: npm test passed 183/183
Not-tested: Manual browser extension flow against a live SUB2API deployment

sidepanel/sidepanel.html

@@ -140,6 +140,11 @@
         <span class="data-label">分组</span>
         <input type="text" id="input-sub2api-group" class="data-input" placeholder="默认 codex" />
       </div>
+      <div class="data-row" id="row-sub2api-default-proxy" style="display:none;">
+        <span class="data-label">默认代理</span>
+        <input type="text" id="input-sub2api-default-proxy" class="data-input"
+          placeholder="shadowrocket 或代理 ID" />
+      </div>
       <div class="data-row">
         <span class="data-label">账户密码</span>
         <div class="input-with-icon">