docs: cover help popover, +/- nav indicators, render_tool_header

User-facing docs (USER-GUIDE en+es, README en+es):
- New short paragraph under §3.1 GUI noting the in-tool Help button
  on every detail page, what it contains (When to use / Steps /
  Examples / Tip), and that content lives in tools.<id>.help_md.
- One-line note in the README tool tables pointing at the same.
- Mention the sidebar +/- nav indicators replacing Streamlit's
  default Material Symbols chevron.

Developer docs:
- DEVELOPER: new "Tool page header" subsection documenting
  render_tool_header(tool_id), the help_md markdown skeleton, and
  the fallback to help.missing_body when a tool's help is absent.
  Update i18n authoring rules to list help.* keys and the per-tool
  help_md field alongside name/description/page_title/page_caption.
- TECHNICAL: new §10c documenting the sidebar nav indicator swap —
  CSS in _HIDE_CHROME_CSS plus _SWAP_NAV_SECTION_INDICATOR_JS
  injected through the hide_streamlit_chrome() iframe bundle.

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

README.md

@@ -18,6 +18,8 @@ Local CSV / Excel cleaning. CLI + browser GUI, no cloud, no install ceremony. GU
 | 08 | Quality Check | Coming Soon |
 | 09 | **Automated Workflows** — chain tools with recommended (not forced) order, save/load JSON, automate weekly cleanups | Ready |
 
+Every tool page has an in-tool **Help** popover (right of the title) with a compact When-to-use / Steps / Examples / Tip card. Copy lives in the language packs (`tools.<id>.help_md`).
+
 ## Download (non-technical users)
 
 Pre-built bundles — no Python install, no admin rights, no internet at runtime. Each release ships two flavors per OS: an **installer** that wires up Desktop + Start Menu / Launchpad shortcuts, and a **portable .zip** you unzip and double-click. Pick whichever your IT policy allows.