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>

docs/DEVELOPER.md

@@ -96,6 +96,36 @@ DeduplicationResult               # deduplicated_df, removed_df, match_groups, l
 
 No other call sites change. Gate auto-discovers it via the registry.
 
+### Tool page header — `render_tool_header(tool_id)`
+
+Every tool page renders its title block via `render_tool_header(tool_id)` in `src/gui/components/_legacy.py` — do not call `st.title()` + `st.caption()` directly. The helper renders:
+
+- `tools.<id>.page_title` as the page title (left column).
+- A **Help** popover button right of the title (icon `:material/help_outline:`, label from `help.button_label`). Clicking opens an `st.popover` containing the markdown body.
+- `tools.<id>.page_caption` as the caption below.
+
+All copy is i18n-driven; editors can tweak help text without touching Python. If a tool is missing its `help_md` key, the popover falls back to `help.missing_body`.
+
+**`help_md` structure** (markdown, stored as a single string with `\n` line breaks in JSON):
+
+```
+**When to use**
+- bullet 1
+- bullet 2
+
+**Steps**
+1. numbered step
+2. numbered step
+
+**Examples**
+- example 1
+- example 2
+
+**Tip** one-sentence pro tip.
+```
+
+Keep it short — the popover is intentionally compact. Mirror the structure across every tool so the muscle memory transfers.
+
 ### i18n — language packs
 
 The GUI's user-facing strings live in `src/i18n/packs/<code>.json`, keyed by ISO-639-1 code. English (`en.json`) is canonical; missing keys in other packs fall back to English, and missing keys in English fall back to the literal dotted key so a typo is visible rather than silent.
@@ -120,7 +150,8 @@ st.warning(t("gate.warning", name=filename))   # {name} interpolated via str.for
 3. Use the dotted key at the call site: `t("section.subsection.key")` or `t("section.key", name=value)` for placeholder interpolation.
 
 **Authoring rules:**
-- Keys live under semantic sections (`home.*`, `upload.*`, `findings.*`, `tools.<id>.name`). Don't nest by language or by tool unless the string is genuinely tool-specific.
+- Keys live under semantic sections (`home.*`, `upload.*`, `findings.*`, `help.*`, `tools.<id>.name`). Don't nest by language or by tool unless the string is genuinely tool-specific.
+- Per-tool header copy lives under `tools.<id>.{page_title, page_caption, help_md}`. `page_caption` is the one-line subtitle under the title; `help_md` is the popover body (see *Tool page header* above). Top-level `help.button_label` / `help.missing_body` are shared across every tool.
 - Use `{named}` placeholders (not positional `{0}`) so translators see what's being interpolated.
 - Strings can contain Streamlit markdown (`**bold**`) — pass through `st.markdown` / `st.caption` as usual.
 - Do **not** put strings inside the farewell-overlay JS payload without going through `_js_html_safe()` in `src/gui/components/_legacy.py`; the helper escapes both the JS string terminator and HTML special chars. The test `TestFarewellEscape` pins that contract.