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/focus_edit.html

@@ -45,9 +45,10 @@
                 </select>
             </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 Changes</button>
-                <a href="/focus/{{ item.id }}" class="btn btn-secondary">Cancel</a>
+                <a href="{{ '/projects/' ~ from_project ~ '?tab=focus' if from_project is defined and from_project else '/focus/' ~ item.id }}" class="btn btn-secondary">Cancel</a>
             </div>
         </div>
     </form>