fix(frontend): style the paywall header and guard undefined class names

The onboarding paywall wrote `dialog-header`, a class no stylesheet
defines, so its close button stacked under the title instead of sitting
beside it. The session-delete confirm buttons had the same problem, and a
`message-entry` wrapper added in August had quietly killed the
`.message + .message` spacing rule in the main chat.

Fixes all three against the patterns already in the codebase, then adds a
contract test so the next undefined class fails instead of shipping: it
scopes itself to our own class families so Tailwind utilities stay out,
strips CSS comments before deciding what counts as defined, and carries
an allowlist of the fifteen deliberate no-op modifiers that must shrink
rather than grow.

Also collapses the paywall's duplicate display heading into the intro
sentence, leaving one heading in the dialog.

BUG-434, BUG-435, BUG-436.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0155nFCgCHtoA7jhSDGmZmMu

frontend/src/components/onboarding-redeem-paywall.tsx

@@ -84,18 +84,15 @@ export function OnboardingRedeemPaywall({
           else keepFocusWithin(event.nativeEvent, event.currentTarget);
         }}
       >
-        <div className="dialog-header">
+        <header className="account-modal-header">
           <h2 id="onboarding-paywall-title">解锁完整咨询</h2>
           <button className="dialog-close" aria-label="关闭" type="button" onClick={close} disabled={pending}>
             <X aria-hidden="true" />
           </button>
-        </div>
+        </header>
         <div className="paywall-intro">
           <span className="paywall-mark" aria-hidden="true"><Gift /></span>
-          <div>
-            <b>入门问题已经准备好</b>
-            <p>填写兑换码，解锁后续咨询、生时校正与完整解读。</p>
-          </div>
+          <p><b>入门问题已经准备好。</b>填写兑换码，解锁后续咨询、生时校正与完整解读。</p>
         </div>
         <div className="redeem-balance">
           <span>当前可用点数</span>