feat: add Materiel.net, Cdiscount and PCComponentes via headless browser

Google/Bing/DuckDuckGo/Qwant were tested as a way to find prices on the
3 remaining requested merchants and all four block scraping immediately
(visible captcha or JS-only anti-bot challenge on the very first
automated request) - not usable as an intermediary. Instead, a headless
browser was used to scrape these 3 merchants directly, since their
blocking (JS-only search, empty body, 403) turned out to be bypassable
with real browser rendering rather than needing to get past a captcha:

- Materiel.net: search is entirely client-side (Stimulus/Turbo), a plain
  GET on /recherche/?query=... always returns the empty "no results"
  template. Fixed by actually filling the search input and clicking
  submit in a real page.
- Cdiscount: returns a fully empty response body to plain HTTP fetches
  (no error, just nothing) but renders fine in a real browser.
- PCComponentes: blocks plain HTTP requests with a 403, but a headless
  browser with a normal user agent goes through.

Implementation:
- New playwright-core dependency, pointed at a system-installed
  Chromium binary via CHROMIUM_EXECUTABLE_PATH (set to /usr/bin/chromium
  here) instead of downloading Playwright's own ~300MB browser
- New headless_browser_service.ts: lazily launches a single shared
  Chromium instance reused across lookups (avoids paying browser launch
  cost per request), with a withPage() helper that always closes its
  browser context afterwards. A new start/browser.ts preload registers
  an app.terminating() hook so the browser process is closed on shutdown
  instead of leaking a child process.
- fetchMaterielNetPrice/fetchCdiscountPrice/fetchPcComponentesPrice
  follow the same model-token matching strategy as the existing
  cheerio-based merchants (LDLC/Grosbill/TopAchat): search on a short
  brand+series query, then pick the result whose label/attribute
  contains the extracted model number, to avoid misreporting an
  unrelated product's price
- PCComponentes exposes clean data-product-price/data-product-category
  attributes directly on result links, so full pre-built "Sobremesa" PCs
  that happen to mention the searched CPU are excluded rather than
  reported as its price
- Fixed two bugs found while testing with a Ryzen 5 9600X (a case with
  multiple search results, unlike the Ryzen 3 3200G single-redirect case
  used earlier):
  - Materiel.net: a search results list page also has .o-product__price
    elements (one per card), so checking for their mere presence isn't
    enough to detect the "redirected to a single product" case; now
    disambiguated by URL pattern (/produit/ vs /recherche/)
  - New parseFlattenedFragmentedPriceText(): Playwright's innerText()
    flattens "<whole>€<sup>cents</sup>" prices to plain strings like
    "284€95" or "2 049€95", which the existing parser misread as
    28495/204995. Cheerio-based scrapers (LDLC/Grosbill) already handled
    this correctly via DOM structure (parseFragmentedPrice); this adds
    the equivalent for already-flattened text.
- findCheapestOffers() now queries all 7 merchants in parallel
- Extended the MerchantOffer type/favicon domain map on the frontend for
  the 3 new merchants

All 30 existing tests still pass unchanged (they don't exercise live
network scraping, only the surrounding auth/validation/caching logic).

package-lock.json

@@ -29,6 +29,7 @@
         "edge.js": "^6.5.1",
         "luxon": "^3.7.2",
         "mysql2": "^3.22.4",
+        "playwright-core": "^1.61.1",
         "reflect-metadata": "^0.2.2",
         "uuid": "^14.0.1",
         "vue": "^3.5.35",
@@ -8097,6 +8098,18 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/playwright-core": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/playwright-core/-/playwright-core-1.61.1.tgz",
+      "integrity": "sha512-h7Qlt6m4REp25qvIdvbDtVmD4LqVXfpRxhORv9L0jzETM05p4fuPJ3dKyuSXQxDSbXnmS79HAgi9589lGSpLkg==",
+      "license": "Apache-2.0",
+      "bin": {
+        "playwright-core": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/pluralize": {
       "version": "8.0.0",
       "resolved": "https://registry.npmjs.org/pluralize/-/pluralize-8.0.0.tgz",