feat(nav): ← Back to Home link on every tool page

Multi-file workflow: a user uploads several files on Home, clicks
"Open <Tool>" on one file's findings, lands on a tool page. The
sidebar lets them get back to Home, but a top-of-page back affordance
is more discoverable and keeps the hand in the same screen region as
the upload list they're working through.

- New ``back_to_home_link()`` helper in components/_legacy.py renders
  a secondary button that calls ``st.switch_page("app.py")`` — under
  ``st.navigation`` that routes to the default (Home) page.
- Wired into every tool page (1-9) directly after
  ``hide_streamlit_chrome()`` and BEFORE the license gate so a Lite
  user who lands on a locked tool can navigate away without paying.
- New i18n key ``nav.back_to_home`` ("← Back to Home" /
  "← Volver al inicio") in en/es packs.

2008 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/i18n/packs/en.json

@@ -149,6 +149,7 @@
     "section_account": "Account",
     "activate_title": "Activate",
     "close_title": "Close",
-    "section_close": "Close"
+    "section_close": "Close",
+    "back_to_home": "← Back to Home"
   }
 }