fix(studio): Welcome layout exception, drop awkward header, Store at slot 2 (v0.39.1)

Three fixes against the v0.39.0 user feedback.

- Layout exception "BoxConstraints forces an infinite
  height". `_PillarRow`'s wide-window path used
  `Row(crossAxisAlignment: CrossAxisAlignment.stretch)`
  inside a `Column` inside a `SingleChildScrollView` — Row
  inherits unbounded height, can't stretch to anything,
  Flutter throws. Wrapped the Row in `IntrinsicHeight` so
  the Row's height is bounded to the tallest child first.
  Same shape `_DocsRow`'s narrow-window path was triggering
  with `Wrap` + `SizedBox(width: double.infinity)` — replaced
  with a plain `Column` for that path; the wide path keeps
  the Wrap with the proper finite cardWidth.

- Dropped the "WIE ES ZUSAMMENPASST" / "HOW IT FITS
  TOGETHER" section header. Operator-feedback was that the
  label sounded like a question without an answer. The hero
  already establishes the page; the three Hub/Module/Flow
  cards are self-explaining beneath it. Header was visual
  clutter and the underlying ARB keys are gone.

- Reordered the sidebar: Welcome, **Store**, Doctor, Flows,
  Audit, Approvals. Store is the operator's daily-use
  surface and belongs above Doctor (which is a diagnostics
  page). Welcome stays slot 0 as the first-launch landing.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_de.arb

@@ -6,7 +6,6 @@
   "navWelcome": "Willkommen",
   "welcomeHeroTitle": "F∆I Platform",
   "welcomeHeroSubtitle": "Deterministische Workflow-Engine für KI-gestützte Dokumentenverarbeitung in regulierten Umgebungen.",
-  "welcomePillarsHeader": "WIE ES ZUSAMMENPASST",
   "welcomePillarHubTitle": "Hub",
   "welcomePillarHubBody": "Ein Rust-Binary, das Module lädt und Flows ausführt. SQLite für den State. Kein Docker, kein Message-Broker, keine Hintergrunddienste, die gepflegt werden müssen.",
   "welcomePillarModuleTitle": "Modul",