feat: add canvas tool panels (multi-grid, upscale, inpaint) and conditional grid mode

Add modal-based tool panels for multi-grid, super-resolution, and inpaint in canvas image-to-image workflow. Grid mode selector only appears for models that support multi-image generation (wan2.7-image, gpt-image-2). Also fixes merge conflict markers in CSS and adds missing toast import.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/AppShell.tsx

@@ -13,6 +13,7 @@ import {
 import { useEffect, useMemo, useRef, useState } from "react";
 import type { ReactNode } from "react";
 import { publicConfigClient, type WebPublicConfig } from "../api/publicConfigClient";
+import { toast } from "./toast/toastStore";
 import type { ServerConnectionHealth } from "../api/serverConnection";
 import { ossAssets } from "../data/ossAssets";
 import { canManageCommunityCases, canReviewCommunity } from "../features/community-review/communityPermissions";