feat(editor): persistent diagnostic strip + run-block + copyable run errors

Three connected fixes that close the loop the analyzer started:

  - **Strip lives at page level, not text-tab**. Moved
    _DiagnosticStrip out of _textTab and into _tabbedBody
    below the TabBarView, so the same error list + quick-fix
    buttons are visible on Graph + Text + Run. The Graph-tab
    operator now sees both the pulsing node AND the message
    explaining what's broken, without flipping tabs. One strip,
    one source of truth.

  - **Run button disabled when there are analyzer errors**.
    FlowEditorController.analyzerErrorCount counts `IssueType.error`
    issues; the action strip's Run button + the RunTab's Start
    button both check it. When > 0:
      · button greys out + icon flips to Icons.block
      · tooltip names the count ('2 errors prevent the run')
      · the Run tab grows an inline red explanation next to
        the disabled Start so the operator isn't left guessing
    Warnings (orange) do NOT block — they're nudges. Operators
    who want to run a half-broken flow during dev can still
    silence the analyzer via the strip.

  - **Run failures are copyable everywhere**. New
    _CopyableErrorBox replaces both the per-run failure detail
    and the per-step failure suffix in the RunTab. Selectable
    monospace + explicit Copy button with 'Copied' feedback.
    Scrollbar-capped at 220 px so the box doesn't push outputs
    off-screen. Mirrors the operator-visible contract:
    'every error is copyable, always.'

Closes the operator-reported regression where a failed run
showed 'Lauf fehlgeschlagen' as plain Text, leaving the
operator unable to paste it into a bug report.

Bumped to 0.19.0.

Signed-off-by: flemming-it <sf@flemming.it>

pubspec.yaml

@@ -1,6 +1,6 @@
 name: fai_studio_flow_editor
 description: Swappable inline YAML editor for F∆I Studio flows.
-version: 0.18.0
+version: 0.19.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor