feat(editor): Fix-dialog from diagnostic strip + inline approval in Run tab

Two big operator-UX wins that the previous batch missed:

1. Diagnostic-strip header gets a 'Beheben' (Fix) button next
   to 'Copy all'. Clicking opens a focused modal listing every
   issue with its quick-fix actions plus an 'apply every quick
   fix' master button. Replaces the cramped expand-strip flow
   for operators who want a deliberate dialog instead of
   scrolling in a 200-px footer.

2. Run tab inlines the Approve / Reject form right under any
   step that the hub paused on system.approval@^0. The
   FlowRunDriver gains three new methods (host-implements
   them; defaults throw with a clear message):

     - pendingApprovalIdForStep(flowName, stepId) → String?
     - approveApproval(approvalId, reviewer) → Future<void>
     - rejectApproval(approvalId, reviewer, reason)

   The _InlineApprovalCard polls the driver for ~3s waiting for
   the hub to materialise the approval row (event-stream race
   against the hub's create), then renders the same form the
   Approvals page does — reviewer + optional reason + Approve /
   Reject buttons. After submit the hub picks up the decision
   on its next poll and emits step.approved / step.rejected,
   which the existing event stream already maps to the right
   step status.

   Closes the operator question 'wie soll das gehen dass da
   Freigaben landen?': they land in the Approvals page AND
   inline in the Run tab; the inline path is now the natural
   workflow.

All 36 editor tests + 24 Studio tests green. Bumped to 0.21.0.

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: 0.20.1
+version: 0.21.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor