feat(gui): inline Help popover next to every tool's title

Adds a contextual Help button on each detail page, right of the title.
Clicking it opens a Streamlit popover with a one-shot how-to: when to
use, numbered steps, before→after examples, and an optional one-line
tip. Designed to be scannable — no paragraph prose.

Implementation:
- New ``render_tool_header(tool_id)`` helper in components replaces the
  bare ``st.title(...) + st.caption(...)`` block on each of the 11 tool
  pages. Title in the wide column, popover in a narrow right column;
  caption sits on its own line beneath.
- Help content is one markdown blob per tool stored in i18n under
  ``tools.<id>.help_md`` (en + es). Editors can tweak copy without
  touching Python.
- ``help.button_label`` and ``help.missing_body`` keys added to both
  packs for the popover trigger and the empty-tool fallback.

All 11 tool pages now use the same header pattern — including the
PDF Extractor and Reconciler which previously had hardcoded title/
caption pairs.

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

src/gui/pages/10_PDF_Extractor.py

@@ -25,6 +25,7 @@ from src.gui.components import (
     hide_streamlit_chrome,
     html_download_button,
     render_sticky_footer,
+    render_tool_header,
 )
 from src.pdf_extract import (
     PdfDependencyMissing,
@@ -103,13 +104,7 @@ def _format_size(n_bytes: int) -> str:
 # Header + dep guard
 # ---------------------------------------------------------------------------
 
-st.markdown("# PDF to CSV")
-st.caption(
-    "Scan bank-statement PDFs for transaction rows "
-    "(``[date] [description] [amount]``). Review the table, uncheck "
-    "rows you don't want, edit any cell that needs fixing, then "
-    "download as CSV. No per-bank setup."
-)
+render_tool_header("10_pdf_extractor")
 
 _pdf_ok, _pdf_missing = _pdf_deps_status()
 if not _pdf_ok: