feat(studio): Welcome page Phase B — embedded doc reader (v0.38.0)

Second slice of the Welcome surface. Operator-facing
documentation now lives inside Studio as bundled assets and
renders inline via a modal sheet — no browser, no external
link, air-gap-tauglich.

- Four operator-readable explainers under `assets/docs/`,
  each with an EN + DE pair:
    architecture[_de].md  — Hub / Module / Flow + how they fit
    security[_de].md       — sandbox model, declared perms,
                              operator ceiling
    audit[_de].md          — hash-chained log, WORM-1 mechanics,
                              `fai admin verify-events`
    flows[_de].md          — flow YAML, templating reference,
                              extract→summarize example
  These are short (≈ 300-500 words each), operator-shaped
  prose. Not copies of the architecture docs in
  fai_platform/docs/architecture/ — those are
  contributor-dense.

- pubspec.yaml declares `assets/docs/` so the markdown ships
  inside the Studio binary. Air-gap deployments read them
  with no network access.

- New `_DocReaderSheet` modal: 85 %-of-viewport bottom sheet,
  drag handle + title bar with the doc icon and close button,
  scrollable Markdown body styled to match Studio chrome.
  Loads `assets/docs/<slug>_<locale>.md` first, falls back to
  the EN file. Locale comes from
  `Localizations.localeOf(context)`.

- `_DocsRow` on the Welcome page sits below the trust-posture
  deck. Two-column grid on ≥ 640 dp, single column below.
  Each card is icon + title + one-line blurb + chevron;
  click opens the reader sheet for that slug.

- 12 new ARB keys for the docs section (header, blurb, four
  card titles + blurbs, close button, error message).
- 4 new icons reused: `account_tree_outlined` (architecture),
  `shield_outlined` (security), `verified_outlined` (audit),
  `alt_route_outlined` (flows).

Implements Phase B of `docs/landing-page-design.md`. Phase C
(live getting-started checklist with persistent state) is the
last remaining slice.

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

assets/docs/flows.md

@@ -0,0 +1,90 @@
+# Flow composition
+
+A flow is a YAML file with three top-level keys: `inputs:`,
+`steps:`, `outputs:`. Read top to bottom, it tells the hub
+what kind of inputs to accept, which modules to call in what
+order, and what to surface as the final result.
+
+## Smallest possible flow
+
+```yaml
+name: hello
+description: Echo a name back as a greeting.
+
+inputs:
+  name:
+    type: text
+
+steps:
+  - id: greet
+    module: debug.echo
+    inputs:
+      payload: "Hello, {{ inputs.name }}"
+
+outputs:
+  greeting: "{{ steps.greet.outputs.payload }}"
+```
+
+`fai run flows/hello.yaml --input name=World` produces:
+
+```json
+{ "greeting": "Hello, World" }
+```
+
+## Real-world example: extract → summarize
+
+The bundled `flows/extract-summarize.yaml` chains two modules:
+
+```yaml
+steps:
+  - id: extract
+    module: text.extract
+    inputs:
+      document: "{{ inputs.file }}"
+
+  - id: summarize
+    module: text.summarize
+    inputs:
+      text: "{{ steps.extract.outputs.text }}"
+      style: "three bullet points"
+```
+
+The `{{ steps.extract.outputs.text }}` template plumbs the
+output of one step into the next. The flow engine type-coerces
+JSON / FileRef / Text per the destination module's manifest, so
+you don't need explicit conversion steps.
+
+## Templating reference (short)
+
+| Expression                         | Resolves to                |
+|------------------------------------|----------------------------|
+| `{{ inputs.X }}`                   | input named X              |
+| `{{ steps.Y.outputs.Z }}`          | output Z of step Y         |
+| `{{ env.OPENAI_API_KEY }}`         | env var (must be declared) |
+| `{{ now }}`                        | ISO-8601 UTC timestamp     |
+| `{{ flow.execution_id }}`          | per-run id (audit-log key) |
+
+## Audit posture
+
+Every step emits four events:
+
+- `step.started` — module name, version, manifest hash
+- `step.completed` — duration, output size, well-known
+  fields auto-elevated (`model_digest`, `engine`, …)
+- `step.failed` — error name + message
+- `flow.completed` — sum of step durations, total events
+
+All hash-chained into the same audit log. See the **Audit-log**
+explainer for the chain mechanics.
+
+## What you'll see in this app
+
+- **Flows page**: list of saved flows under `~/.fai/data/flows/`.
+  Click `Run`, fill any required inputs, watch the result.
+- **Audit page**: per-step events appear within seconds of a
+  flow finishing. Filter on `flow.` to see only flow-level
+  rows.
+- **Store**: any installed module shows up as available for
+  `module:` in a flow YAML. Synthetic federated entries
+  (`mcp.<server>.<tool>`, `n8n.<endpoint>.<workflow>`) work
+  the same way.