A tile whose owned query is missing or shared cannot be removed from any surface

Problem

removeDashboardPanel proves ownership before deleting anything and refuses
ownership-unproven when the tile's queryId names no query in
workspace.queries, or names one with more than one owner
(src/dashboard/application/dashboard-removal.ts:128-131). Both the Dashboards
tree and — since #537 — the tile header route their delete through it.

That is the correct fail-closed behaviour #494/#537 specify. The consequence is
that a tile whose query is missing (a dangling queryId) or shared by a
second tile now has no removal path from any surface:

• the Dashboards-tree trash is disabled with a reason;
• the tile header's ⋯ → Remove tile row is disabled with the same reason;
• and #537 deleted the last escape hatch — the tile head's old document-only
  remove-tile dispatch, which removed such a tile without touching queries.

How to reach it

A queries-only bundle import (#448) or a corrupt aggregate can produce a tile
pointing at an id no query carries. A legacy or hand-edited workspace can carry
two tiles referencing one query, which #427 forbids reaching but does not
retroactively repair.

Why it was deferred

#537's acceptance criteria require exactly this fail-closed behaviour, so the
alternative belongs in its own decision rather than smuggled into that change.

Options

1. A separate, explicitly-labelled "Remove tile only" action, offered only
   when removal is refused because the query is missing — a dangling reference has
   no second resource to be atomic about. Would keep the remove-tile command,
   which currently has no UI dispatcher left.
2. Give removeDashboardPanel an explicit tile-only arm for the missing-query
   case, so one path still covers it.
3. Accept it, and handle these workspaces through Import/Replace only.

Option 1 or 2 seems right for the missing-query case; a shared query arguably
should stay unremovable, since either tile still renders it.

Start in src/dashboard/application/dashboard-removal.ts:128-131, then trace the Dashboards-tree trash and tile-header Remove tile routes, including the existing remove-tile command. Decide which documented option applies for missing queries while preserving the refusal for shared queries. Done means a chosen removal path works for dangling tiles from the relevant surface without deleting a query.