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/gui/pages/6_Outlier_Detector.py

@@ -12,12 +12,14 @@ if str(_project_root) not in sys.path:
     sys.path.insert(0, str(_project_root))
 
 from src.gui.components import (
+    back_to_home_link,
     hide_streamlit_chrome,
     require_feature_or_render_upgrade,
 )
 from src.license import FeatureFlag
 
 hide_streamlit_chrome()
+back_to_home_link()
 require_feature_or_render_upgrade(FeatureFlag.OUTLIER_DETECTOR)
 
 # ---------------------------------------------------------------------------