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/lib/i18n/en.ts

@@ -27,6 +27,7 @@ export const en = {
   'boot.errorBody': 'Please try again in a moment.',
 
   'launch.errorTitle': "Can't open in Telegram",
+  'launch.errorTitleVk': "Can't open in VK",
   'launch.errorBody': 'Screenshot or share this with the developer.',
   'launch.share': 'Share',
   'launch.copied': 'Copied',

ui/src/lib/i18n/ru.ts

@@ -28,6 +28,7 @@ export const ru: Record<MessageKey, string> = {
   'boot.errorBody': 'Попробуйте ещё раз или зайдите позже.',
 
   'launch.errorTitle': 'Не открывается в Telegram',
+  'launch.errorTitleVk': 'Не открывается в VK',
   'launch.errorBody': 'Сделайте скриншот или поделитесь с разработчиком.',
   'launch.share': 'Поделиться',
   'launch.copied': 'Скопировано',