fix(extension): normalize FC URL so credential push doesn't 405

The stored apiUrl was required to already carry the `/api` suffix, since
api.js builds requests as `${baseUrl}/credentials`. The options label read
"FC base URL", so entering the instance root -- the natural reading --
sent every request one path segment short: POST /credentials hit the Vue
SPA catch-all and came back 405, and GET /extension/manifest 404'd.

Worse, Test Connection reported success on it: the catch-all answers GET
/credentials with 200 HTML, so `r.ok` was true and the only affordance
meant to catch this misconfiguration actively masked it.

Normalize instead of validate (rules 92, 26):

- New lib/url.js: normalizeApiUrl / webRootFromApiUrl, one source shared
  by the background client and the options page. Accepts either the
  instance root or the API root.
- api.js normalizes on read, so configs already stored in the broken form
  heal themselves without the operator reopening Settings.
- options.js stores the canonical form, echoes back what it saved, and
  the test now asserts a JSON content-type -- killing the false green.
- 404/405 in request() now names the URL and points at the setting.
- Options label/placeholder state that both forms work.

Version 1.0.9 -> 1.0.10 in BOTH manifest.json and package.json; build.yml
resolves the release version from package.json, and a stale value there
would hit the cached ext-1.0.9 asset and republish the old XPI unsigned
against the new code.

Refs #2393

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

extension/package.json

@@ -1,6 +1,6 @@
 {
   "name": "fabledcurator-extension",
-  "version": "1.0.9",
+  "version": "1.0.10",
   "private": true,
   "description": "Firefox extension for FabledCurator",
   "scripts": {