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/i18n/packs/en.json

@@ -177,7 +177,7 @@
       "description": "Pull transactions out of bank-statement PDFs into a clean CSV file.",
       "page_title": "PDF to CSV",
       "page_caption": "Pull transactions out of bank-statement PDFs into a clean CSV file.",
-      "help_md": "**When to use**\n- Bank or credit-card statements\n- Vendor invoices with line-item tables\n- Any PDF with a transaction table\n\n**Steps**\n1. Upload a PDF\n2. Draw a box around the table (once per source)\n3. Save the template (e.g. `Chase Checking`)\n4. Reuse the template on every future statement of that type\n5. Export the CSV\n\n**Examples**\n- Chase March statement → 87 transactions extracted\n- Same template auto-runs on April, May, June\n- Batch mode: process 12 months at once\n\n**Tip** Templates are per source (Chase, Wells Fargo, …). Build one per source you receive regularly."
+      "help_md": "**When to use**\n- Bank or credit-card statements\n- Vendor invoices with line-item tables\n- Any PDF with a transaction table\n\n**Steps**\n1. Upload one or more PDFs (batch is fine)\n2. Click **Scan** — rows that look like transactions are pulled out automatically\n3. Uncheck any rows you don't want\n4. Pick your date format (and turn on OCR if the PDF is a scanned image)\n5. Download the CSV\n\n**Examples**\n- Chase March statement → 87 transactions found\n- Drop in 12 months at once and get one combined CSV\n- Image-only scan + OCR → still works if Tesseract is installed\n\n**Tip** If a withdrawal shows as `(4.50)`, leave **Treat (4.50) as negative** on. Turn it off only if your statements use a different convention."
     },
     "11_reconciler": {
       "name": "Reconcile Two Files",