chore: production-readiness sweep on the help-popover wave

- Drop unused 'from src.i18n import t' from pages 1-9 (the swap to
  render_tool_header(tool_id) means no page calls t() directly anymore).
  Pages 10, 11 and the underscore-prefixed pages were already clean or
  legitimately use t().

- Rewrite PDF Extractor help_md (en + es). The original prose described
  features the tool does NOT have — template drawing, per-source saved
  templates, automatic reuse. The actual tool is a heuristic batch
  scanner (per its own docstring: "No templates, no per-bank
  configuration"). New copy: scan → uncheck → pick date format → enable
  OCR if needed → download. Spanish version tagged with
  '<!-- TODO: review Spanish -->' since the prose is best-effort.

- Document why both stSidebarNavSectionHeader (legacy, streamlit~=1.35)
  and stNavSectionHeader (current, 1.57) testids appear in the chrome
  CSS — requirements floor is streamlit>=1.35,<2 so dropping the legacy
  selector would silently break the lower bound.

- Pin the t()-returns-key-on-miss contract that render_tool_header's
  fallback path depends on, with a comment at the call site.

- Pin the demo's intentional skip of hide_streamlit_chrome (so the
  +/- sidebar swap JS doesn't ever try to load there) with a load-
  bearing comment in app_demo.py.

- Confirmed i18n parity: every tool id has page_title / page_caption /
  description / name / help_md in BOTH packs; help.button_label and
  help.missing_body in both.

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

src/gui/pages/9_Pipeline_Runner.py

@@ -23,7 +23,6 @@ from src.gui.components import (
     pickup_or_upload,
     require_feature_or_render_upgrade,
 )
-from src.i18n import t
 from src.core.pipeline import (
     Pipeline,
     SOFT_DEPENDENCIES,