feat: surface missing models in Error Tab for OSS and remove legacy dialog (#9921)

## Summary
- Surface missing models in the Error Tab for OSS environments,
replacing the legacy modal dialog
- Add Download button per model and Download All button in group header
with file size display
- Move download business logic from `components/dialog/content` to
`platform/missingModel`
- Remove legacy missing models dialog components and composable

## Changes
- **Pipeline**: Remove `isCloud` guard from `scanAllModelCandidates` and
`surfaceMissingModels` so OSS detects missing models
- **Grouping**: Group non-asset-supported models by directory in OSS
instead of lumping into UNSUPPORTED
- **UI**: Add Download button (matching Install Node Pack design) and
Download All header button
- **Store**: Add `folderPaths`/`fileSizes` state with setter methods,
race condition guard
- **Cleanup**: Delete `MissingModelsContent`, `MissingModelsHeader`,
`MissingModelsFooter`, `useMissingModelsDialog`, `missingModelsUtils`
- **Tests**: Add OSS/Cloud grouping tests, migrate Playwright E2E to
Error Tab, improve test isolation
- **Snapshots**: Reset Playwright screenshot expectations since OSS
missing model error detection now causes red highlights on affected
nodes
- **Accessibility**: Add `aria-label` with model name, `aria-expanded`
on toggle, warning icon for unknown category

## Test plan
- [x] Unit tests pass (86 tests)
- [x] TypeScript typecheck passes
- [x] knip passes
- [x] Load workflow with missing models in OSS → Error Tab shows missing
models grouped by directory
- [x] Download button triggers browser download with correct URL
- [x] Download All button downloads all downloadable models
- [x] Cloud environment behavior unchanged
- [x] Playwright E2E: `pnpm test:browser:local -- --grep "Missing models
in Error Tab"`

## Screenshots


https://github.com/user-attachments/assets/12f15e09-215a-4c58-87ed-39bbffd1359c

 


┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9921-feat-surface-missing-models-in-Error-Tab-for-OSS-and-remove-legacy-dialog-3236d73d365081f0a9dfc291978f5ecf)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: github-actions <github-actions@github.com>

Author: 3 people

browser_tests/fixtures/ComfyPage.ts

@@ -431,9 +431,9 @@ export const comfyPageFixture = base.extend<{
         // Disable toast warning about version compatibility, as they may or
         // may not appear - depending on upstream ComfyUI dependencies
         'Comfy.VersionCompatibility.DisableWarnings': true,
-        // Browser tests should opt into missing-model warnings explicitly so
-        // workflows do not render differently based on models present on disk.
-        'Comfy.Workflow.ShowMissingModelsWarning': false
+        // Disable errors tab to prevent missing model detection from
+        // rendering error indicators on nodes during unrelated tests.
+        'Comfy.RightSidePanel.ShowErrorsTab': false
       })
     } catch (e) {
       console.error(e)