v0.250.0: i18n slice 1 release C — the language switch on every dashboard page (R-556)

- addLanguageData: the v0.247.0 hide condition (switch only on non-Hungarian pages or with ?lang=) is
  deleted; every template is converted, so the offer no longer leads to a half-English page.
- 89 layout parity fixtures re-captured; each equals its predecessor plus exactly one switch form after
  the version span (checked byte-for-byte, 89 of 89). The 17 standalone fixtures are unchanged.
- TestLanguageSwitch_EndToEnd: a Hungarian household with no ?lang= sees the form (red-proofed by
  restoring the condition: this test and 89 parity cases fail).
- CHANGELOG v0.250.0, README §18, CONTEXT.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/web/testdata/i18n_parity/settings_security_full.html

@@ -147,7 +147,7 @@
                 <li><a href="/settings/security" class="active"><svg class="ico"><use href="#i-lock"/></svg>Biztonság és hozzáférés</a></li>
             </ul>
             <div class="sidebar-footer">
-                <span class="version">0.247.0</span>
+                <span class="version">0.247.0</span><form method="POST" action="/settings/language" class="lang-switch"><input type="hidden" name="back" value="/i18n-fixture"><button type="submit" name="lang" value="hu" class="lang-switch-btn active" lang="hu">Magyar</button><button type="submit" name="lang" value="en" class="lang-switch-btn" lang="en">English</button></form>
                 <a href="/logout" class="logout-link">Kijelentkezés ↗</a>
             </div>
         </div>