fix(ui): roll back refused actions, own background work, unify dialogs

[mrsibe]

What does this PR do?

Fixes a group of interaction/lifecycle bugs that share one shape: a UI surface
kept state that no longer matched what the main process had actually done. It
also consolidates the duplicated confirmation dialogs and replaces the last
native alert()s.

This is cleanup on top of the worker + background-ingestion work (#176), not a
change to that architecture.

Why?

• continueMessage() marked the message pending and registered the live turn
  before the IPC. Main refuses past the continuation bound, and the invoke can
  reject as well — either left the message showing "generating" forever.
• SettingsDialog only seeded pendingSettings / pendingConnections on
  mount, and X / Esc / overlay closed it without discarding them, so unsaved
  edits came back on reopen.
• File imports were moved to the background queue, but refreshAfterImport
  still cleared isIndexing, so the top progress vanished while embedding was
  still running. URL / pasted text / notes still blocked their dialog for the
  whole index, which made them inconsistent with file imports.
• Confirmation dialogs were four near-copies split across Dialog and
  AlertDialog; a few native alert()s disagreed with the global Sonner style;
  and the Quiz / Anki config dialogs disagreed on reset-on-close.

What changed?

• chat: roll back the optimistic continue state (turn, message status,
  streaming pointer, assembler, sequence) when Main refuses or the invoke
  rejects.
• settings: route every close path through one handleClose() that discards
  pending settings/connections; add the Radix DialogTitle /
  DialogDescription.
• knowledge: replace isIndexing + indexProgress with indexingJobs
  keyed by documentId, fed by per-document progress broadcasts.
  refreshAfterImport no longer clears the queue, and the top bar shows
  "Indexing N sources · X%".
• knowledge: URL, pasted text and notes now register a pending source and
  index in the background queue like file imports; the Add Source dialog only
  tracks the short isSubmitting registration.
• dialogs: collapse ConfirmDialog, DeleteConfirmDialog,
  DeleteNoteConfirmDialog and UnsavedChangesDialog into one
  ConfirmActionDialog on AlertDialog; replace the remaining native
  alert() with Sonner; Quiz / Anki config discard input on close and reset on
  submit.
• floating layers: dismiss the session switcher and the library Add menu on
  an outside press via a document listener. The full-viewport overlays lost to
  sibling position: relative cards, and the one inside a
  -webkit-app-region: drag header had its clicks eaten by window dragging.
• chat: inset the bottom scroll fade by the scrollbar track so it no longer
  paints over the scrollbar.
• scroll area: override Radix's inner display: table wrapper to block,
  so a truncate list row can no longer outgrow the viewport and cover the
  library's right padding at the panel's minimum width.

How was this tested?

• The repo's Verify workflow runs npm run typecheck, npm run check:design,
  npm test (478/478), npm run build:unpack and npm run smoke:packaged on
  Ubuntu, macOS and Windows.
• Manual: continue-generation refusal, Settings close/reopen with unsaved edits,
  background file-import progress, URL / text / note import, outside-click
  dismissal of the session switcher and the library menu, and the library at its
  260px minimum width.

Screenshots / recordings

Not captured in this environment; the affected surfaces are the chat panel, the
Settings dialog, the library list/menu and the session switcher.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow.
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable