feat: remove stopped ingest tasks from the queue

[annieyii]

Closes #707

What

Lets the user permanently remove stopped ingest tasks from the queue, one at a
time from the task row or in bulk from the existing selection bar.

• removeTasks(taskIds) in ingest-queue.ts drops failed / cancelled tasks
  for the active project in a single queue write. removeTask(taskId) is a thin
  wrapper over it.
• Pending and processing tasks are skipped, not force-stopped. Cancel them first,
  then remove. Nothing touches activeRuns, cancelledInFlightTaskIds or the
  commit coordinator, so an in-flight run keeps owning its own teardown.
• Tasks with a live entry in activeRuns are skipped too. cancelled only
  records that cancellation was requested: the run holds its worker slot until
  it observes the AbortSignal, and a commit past the interruptible point still
  finishes. Removing the entry then would free the sourcePath that enqueueBatch
  dedupes against, so re-importing the same source would mint a fresh task ID
  that the !activeRuns.has(task.id) guard in processNext cannot match. The
  two runs cannot interleave their writes, since autoIngest holds a per-source
  project lock, but the newcomer would take a worker slot and sit in the panel
  as "processing" while it blocks on that lock; with a small worker pool the
  queue can look stalled while nothing is able to progress.
• hasActiveRun(taskId) lets the panel hide the remove affordance for that beat
  rather than offering a click the core would refuse, which also keeps the bulk
  confirmation count honest.
• retainQueuedSelection() reconciles the selection against the queue after a
  removal instead of clearing it, so a row that was not removed stays visible
  and selected. It returns the original Set when nothing changed.
• Activity Panel: a trash button on every removable stopped row (hidden while
  a run is still winding down), and a "Remove" action in the bulk bar next to
  Restart / Cancel. The bulk action confirms first because the removal cannot be
  undone.
• Two new key paths, common.remove and activity.removeSelectedConfirm, added
  across all four bundles. The row button previously reused common.dismiss,
  which translates to "hide" (关闭, Nascondi, Скрыть) and misdescribes a
  permanent action.

Why

#557 established that stopped tasks are retained on purpose so they can be
restarted, and 0fd9257 implemented that by turning cancel from "drop the task"
into "keep it as cancelled". That closed the recoverability gap but left no way
out for tasks that will never succeed: a corrupt PDF, an unsupported format, or a
source file the user has since deleted. A successful task leaves the queue as
soon as it commits, and saveQueue filters any stray done entry, so nothing
else accumulates. failed and cancelled entries stay in the Activity Panel
indefinitely. #707 reports exactly this.

Two removal paths already exist and neither fits. discardTasksForSources is
reachable only through scheduled-import reconciliation, never from the UI.
clearCompletedTasks drops every non-pending/processing task in one call, has no
production caller, and carries neither a selection nor a live-run guard, so
exposing it as-is would give up the granularity #707 asks for.

Relationship to the other open queue-cleanup PRs

	scope	statuses
#690	clear the whole queue, aborts in-flight work	all
#704	bulk clear, part of a broad multi-feature PR	cancelled only
this	single row + bulk over the existing selection	failed + cancelled

This one is the narrowest of the three: it never stops running work, and it
reuses the selection bar that #557 already added rather than introducing another
top-level button.

Worth flagging: #704 also adds an export async function removeTasks(taskIds: readonly string[]): Promise<number> immediately after discardTasksForSources,
so the two cannot be merged mechanically even though the behaviour differs
(#704 accepts cancelled only and has no live-run guard). #690 touches the same
seven files and currently shows as conflicting. Happy to rebase, rename, or fold
this into whichever of the others you prefer to land first.

Tests

src/lib/ingest-queue.test.ts:

• removes a cancelled task, refuses a processing one
• removes a failed task and asserts the persisted snapshot holds exactly the
  survivor, not merely that the removed ID is absent
• bulk removal over a mixed selection removes only the stopped task; unknown IDs
  and an empty selection are no-ops
• a cancelled task whose run has not exited yet is refused
• retainQueuedSelection drops removed IDs, keeps refused and untouched ones,
  and preserves Set identity when nothing changed

Mutation checks: dropping the activeRuns guard turns the active-cancelled test
red, persisting an empty snapshot turns the survivor test red, and clearing the
selection outright or removing the Set identity short-circuit each turn the
reconciliation test red.

npm run test:mocks → 132 files, 1881 tests passing. npm run typecheck and
npm run build clean. The i18n parity test covers both new key paths.