v3.27.0 review: the outfile import hardening

- The endpoint validates algorithm16 (3des2/aes, case-insensitive) and mode
  (merge/replace or omitted): a misspelled value answers 400 instead of
  silently picking 3DES2 or turning an import into a dry run.  The choices
  come from outfile.ALGORITHM16_CHOICES, derived from the algorithm map, so
  the validation and the parser cannot drift apart.
- A present but unusable PIN/PUK value is reported with its raw text: a value
  that strips to nothing (e.g. 'abcd') used to vanish without a report line,
  contradicting the "nothing is silently dropped" contract.  A column absent
  from a short row stays silent - it is not present.
- The column-line detection drops the arbitrary 8-character column-name cap:
  a candidate header must carry ICCID plus at least one column the import
  consumes (a fixed name or a numbered KIC/KID), so a long unknown column
  name next to a real one parses while 'ICCID <value>' and
  'ICCID CUSTOMER_NAME' config lines are still rejected.  Both real samples
  are unaffected.
- PWA: a chosen default keyset survives an empty or failed preview (it used
  to reset to null, and the import silently fell back to the card's first
  keyset); the select renders the preview's keysets plus a remembered value.
- PWA: the destructive Replace mode asks for confirmation first, matching the
  other store-wiping actions.
- api.md documents both validations.  Tests: the endpoint's 400s and the
  replace import, the raw-value report, the header guards, the keyset
  preservation and the declined confirmation.  906 frontend + 904 Python;
  sw cache simple-v437.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v436';
+const CACHE = 'simple-v437';
 const URLS = [
   'index.html',
   'help.html',