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/task_form.html

@@ -108,10 +108,11 @@
             {% if prefill_parent_id %}
             <input type="hidden" name="parent_id" value="{{ prefill_parent_id }}">
             {% endif %}
+            {% 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 Changes' if item else 'Create Task' }}</button>
-                <a href="{{ '/tasks/' ~ item.id if item else '/tasks' }}" class="btn btn-secondary">Cancel</a>
+                <a href="{{ '/projects/' ~ from_project ~ '?tab=tasks' if from_project is defined and from_project else ('/tasks/' ~ item.id if item else '/tasks') }}" class="btn btn-secondary">Cancel</a>
             </div>
         </div>
     </form>