feat: return-to-project redirects from create/edit forms

When creating or editing items from a project detail tab, users now
return to that project's tab instead of the entity's own page.
Edit links pass from_project param; forms include hidden field.
Reassigning to a different project redirects to the new project.
Decisions/meetings create from project context inserts junction rows.
File uploads from project context redirect back to project files tab.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/contact_form.html

@@ -46,7 +46,8 @@
         </datalist>
     </div>
 
-    <div class="form-actions"><button type="submit" class="btn btn-primary">{{ 'Save' if item else 'Create' }}</button><a href="{{ '/contacts/' ~ item.id if item else '/contacts' }}" class="btn btn-secondary">Cancel</a></div>
+    {% if from_project is defined and from_project %}<input type="hidden" name="from_project" value="{{ from_project }}">{% endif %}
+    <div class="form-actions"><button type="submit" class="btn btn-primary">{{ 'Save' if item else 'Create' }}</button><a href="{{ '/projects/' ~ from_project ~ '?tab=contacts' if from_project is defined and from_project else ('/contacts/' ~ item.id if item else '/contacts') }}" class="btn btn-secondary">Cancel</a></div>
 </div>
 </form></div>