Files: legible scope chips, honest upload confirmation, bottom-right toast, full-height detail pane

[TheGreatAxios]

Summary

Four owner-reported fixes on the Files page (CL-6563):

• Scope chips. The top bar showed "All" / "New Workbench" / "All workbenches" as three undifferentiated buttons. "All" was never a kind or scope filter — it's a clear-selection action (close the open file, return to the list), only rendered while a file is selected. It's relabeled "Back to files" so it can't be mistaken for a filter option. The real scope control — "New Workbench" vs. "All workbenches" — is now visually grouped as one bordered segmented control (the same idiom ViewToggle already uses for rows/grid in this same bar), so it reads as one control answering one question. Checked Skills, Agents, Routines, and Insights for the same pattern: none of them have it — Insights has its own, already-legible workbench-scope switcher (a different pattern). The ambiguous row was unique to Files.
• Upload confirmation. A completed upload now shows a toast naming the title the server actually stored, not the local file name the browser sent — grounded in the real upload response, since a sibling lane is separately fixing a bug where uploaded content can read back empty. A failed upload still surfaces the failure instead of a fake success toast.
• Toast position. The app's one <Toaster> mount (main.tsx) now passes position="bottom-right". A hardcoded centering rule in the vendored @corbits/react-ui stylesheet fights any non-center position with !important; a higher-specificity override in app.css cancels it for the right-position case. The existing "one toast system" invariant test (CL-6372) is untouched and still passes.
• Detail pane height. The file preview pane now fills the available height instead of floating as a short panel in an otherwise empty page.

Test plan

• apps/web/test/library-page-selection.test.tsx — new coverage for the grouped scope control, the relabeled clear-selection action, and the full-height detail pane
• apps/web/test/library-upload-confirmation.test.tsx — new: a completed upload confirms the server's title, a failed upload surfaces the error and never fakes a success toast
• apps/web/test/toast-single-system.test.tsx — new case pinning the app's Toaster to bottom-right; existing CL-6372 invariant tests untouched and passing
• Full apps/web test suite (840 tests), typecheck, lint, and check:tailwind-source all pass

https://linear.app/abklabs/issue/CL-6563/files-page-legible-scope-chips-honest-upload-confirmation-bottom-right