feat(studio): brand-agnostic onboarding hints + clickable checklist + theme-picker clarity

Three changes:

1. Onboarding checklist hints no longer mention DeepWiki,
   Semgrep, text.extract, or extract-summarize.yaml by
   name. Brand- and file-specific copy moved to neutral
   feature pointers ('add an MCP source', 'install a
   module'). Brand names belong on the welcome page only
   if they're load-bearing for understanding, which they
   aren't.

2. Checklist rows are now clickable. Tapping a row navigates
   to the page where the operator can complete it — MCP +
   AI open Settings, Module opens Store, Flow opens Flows.
   StudioShellState gains a public navigateTo(pageId) helper
   so descendants can drive sidebar selection without
   plumbing a controller through props.

3. Theme picker hint copy rewritten. Old hint suggested
   'Custom' was a single-colour tweak; new copy spells out
   that Material 3 derives a full palette (primary,
   secondary, tertiary, surface, …) from one seed colour.
   The Custom tile shows a small tune icon so its
   open-the-picker semantics is visually distinct from the
   apply-immediately plugin tiles.

Editor bumped to 0.12.0 (panel toggle + LabVIEW-style port
colours).

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

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.58.0
+version: 0.59.0
 
 environment:
   sdk: ^3.11.0-200.1.beta