feat(vk): show a launch diagnostic on a direct /vk/ open instead of a guest

Opening the dedicated /vk/ entry directly in an ordinary browser (no signed VK
launch) fell through to the web flow and silently started a throwaway guest,
which is wrong for a VK-only entry. Mirror the existing /telegram/ launch-error
behaviour: render a compact, shareable, privacy-safe diagnostic screen instead.

- Boot: a new branch `onVKPath() && !insideVK()` sets app.launchError and stops
  the fall-through, the VK counterpart of the /telegram/ diagnostic guard.
- Diagnostic (passive, no VK Bridge round-trip): reads the URL launch-parameter
  NAMES (never the `sign` value — auth material), whether the URL was signed,
  `vk_platform`, the iframe/referrer context, plus the shared client-environment
  lines. VK signs the launch URL at load, so — unlike Telegram's initData — the
  parameters never arrive late; hence the VK screen offers Share only, no Retry.
- Generalise the screen: TelegramLaunchError.svelte -> LaunchError.svelte, which
  renders a neutral pre-formatted report and a per-platform title, with Retry
  gated on a `retry` flag (Telegram true, VK false). app.launchError is now a
  neutral LaunchDiag { platform, report, retry }.
- New lib/launchdiag.ts holds the shared pieces both platforms reuse: the
  LaunchDiag shape, the client-environment lines, and the query field-NAME
  reader (moved out of telegram.ts so VK does not duplicate them).

Tests: pure vkDiagLines units, incl. a guard that the `sign` value never leaks
into the report. i18n: launch.errorTitleVk (en/ru). Docs: FUNCTIONAL (+ru) user
story and ARCHITECTURE entry-path note.

ui/src/screens/LaunchError.svelte

@@ -1,34 +1,20 @@
 <script lang="ts">
-  // Shown on the dedicated /telegram/ entry when a Mini App launch carried no sign-in data (empty
-  // initData) — instead of bouncing the visitor to the marketing landing. It states the problem
-  // plainly and renders a compact, privacy-safe diagnostic snapshot (taken at the moment of
-  // failure, app.launchError) sized to fit one screenshot, with a Share button (the OS share sheet,
-  // or a clipboard copy on desktop) so a stuck user can send it to the developer to pinpoint why
-  // Telegram provided no initData — notably on some Android clients. The snapshot carries only
-  // presence / identification signals and field NAMES, never the signed initData itself, nor an IP.
+  // Shown on a dedicated Mini App entry (/telegram/ or /vk/) opened without a valid launch — instead of
+  // bouncing to the marketing landing (Telegram) or silently proceeding as a guest (VK). It states the
+  // problem plainly and renders a compact, privacy-safe diagnostic snapshot (app.launchError, taken at
+  // the moment of failure) sized to fit one screenshot, with a Share button (the OS share sheet, or a
+  // clipboard copy on desktop) so a stuck user can send it to the developer to pinpoint why the launch
+  // carried no sign-in data. The snapshot carries only presence / identification signals and field
+  // NAMES, never the signed launch data itself, nor an IP. Retry is shown only where a late-arriving
+  // launch can still succeed (Telegram initData); VK delivers its signed parameters in the URL at load,
+  // so it offers Share only.
   import { app, retryTelegramLaunch } from '../lib/app.svelte';
   import { shareText } from '../lib/share';
   import { t } from '../lib/i18n/index.svelte';
 
   const diag = $derived(app.launchError);
-
-  // One compact "key: value" line per fact; the field labels are fixed diagnostic tokens (not UI
-  // prose), so the developer reads the same report in any locale. Kept terse to fit one screenshot.
-  const report = $derived(
-    diag
-      ? [
-          `sdk-load: ${diag.sdkLoad}`,
-          `sdk: ${diag.hasSDK ? 'yes' : 'no'}   webapp: ${diag.hasWebApp ? 'yes' : 'no'}`,
-          `tg-platform: ${diag.platform || '—'}   tg-version: ${diag.version || '—'}`,
-          `initData: ${diag.initDataLen > 0 ? diag.initDataLen : 'empty'}   tgdata-in-url: ${diag.hashHadTgData ? 'yes' : 'no'}`,
-          `fields: ${diag.fieldsPresent.join(',') || '—'}`,
-          `missing: ${diag.fieldsMissing.join(',') || '—'}`,
-          `os: ${diag.osPlatform || '—'}   mobile: ${diag.mobile || '—'}`,
-          `browser: ${diag.browser || '—'}`,
-          `ua: ${diag.userAgent || '—'}`,
-        ].join('\n')
-      : '',
-  );
+  // The title names the platform the entry belongs to; the body and the report are platform-neutral.
+  const title = $derived(t(diag?.platform === 'vk' ? 'launch.errorTitleVk' : 'launch.errorTitle'));
 
   let retrying = $state(false);
   async function retry(): Promise<void> {
@@ -44,7 +30,8 @@
   let copied = $state(false);
   let copyTimer: ReturnType<typeof setTimeout> | undefined;
   async function share(): Promise<void> {
-    const r = await shareText(report, t('launch.errorTitle'));
+    if (!diag) return;
+    const r = await shareText(diag.report, title);
     // The OS share sheet is its own feedback; a desktop clipboard copy is silent, so confirm it.
     if (r === 'copied') {
       copied = true;
@@ -57,12 +44,14 @@
 {#if diag}
   <div class="boot">
     <div class="card">
-      <h1>{t('launch.errorTitle')}</h1>
+      <h1>{title}</h1>
       <p class="msg">{t('launch.errorBody')}</p>
-      <pre class="diag">{report}</pre>
+      <pre class="diag">{diag.report}</pre>
       <div class="actions">
         <button class="share" onclick={share}>{copied ? t('launch.copied') : t('launch.share')}</button>
-        <button class="retry" onclick={retry} disabled={retrying}>{t('common.retry')}</button>
+        {#if diag.retry}
+          <button class="retry" onclick={retry} disabled={retrying}>{t('common.retry')}</button>
+        {/if}
       </div>
     </div>
   </div>