fix(editor): drop misaligned gutter error pin, add bottom diagnostic strip

Two glitches visible in 0.15.1:

  1. The flutter_code_editor gutter renders an Icons.cancel pin
     for every analyzer issue, hard-positioned inside a 16-px
     slot. With a single-digit indent the icon centres look off;
     with double-digit line numbers it shifts further.
  2. The hover popup for that pin paints at `offset.dx + width`
     of the gutter cell — which lands INSIDE the code area, so
     'Unknown capability ...' overlaps the YAML text the
     operator is trying to read.

The package doesn't expose a fix for either — the icon is a
const in error.dart, the popup position is hard-coded in the
StatefulWidget. Cleanest path: hide the gutter error column
entirely (`GutterStyle.showErrors: false`) and surface the
analyzer issues through a dedicated bottom strip we own.

The new _DiagnosticStrip renders as a single 22-px line under
the editor: a coloured dot + 'N errors · M warnings · L7:
${first.message}'. Tap expands into a per-issue list capped at
140-px scroll, dot-coloured by IssueType. Empty state shows a
muted 'No issues' so the strip never disappears and never
shifts the layout.

Wavy underlines in the text body keep working untouched — the
two surfaces complement each other: the underline points at
the broken token, the strip names what's wrong.

Bumped to 0.16.0.

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: 0.15.1
+version: 0.16.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor