Fix missing drop-indicator styling for in-column block drag reordering

[brylie]

Dragging a block to reorder it within (or into) a column shows no visible drop-indicator line during the drag — the reorder itself still works (it's driven by component state, not CSS), but the drag feedback silently disappears, unlike the polished accent-colored indicator the Document's top-level flow shows for the same interaction.

The root cause is a Svelte scoped-CSS mismatch: .drop-indicator's only style definition lives in src/routes/space/[spaceId]/doc/[id]/+page.svelte's own <style> block. Svelte's automatic style scoping stamps a per-component hash class onto both that rule and every element +page.svelte's own template renders — but src/lib/components/ColumnsBlock.svelte renders its own <div class="drop-indicator"> at two spots (between column blocks, and after the last one) from a different component with no <style> block of its own, so the compiled .drop-indicator.svelte-xxxxx selector never matches those elements.

This predates #239 (present since #148 introduced ColumnsBlock.svelte) and was noticed but deliberately left unchanged while extracting BlockRow.svelte in #239, since that PR's scope excluded drop-indicator rendering and any behavior change.

• .drop-indicator renders visibly wherever it's used inside a column — either promote the rule to :global(.drop-indicator) in ColumnsBlock.svelte, or duplicate the rule in a <style> block there.
• Manually verify (or add a rendered-style assertion) that dragging a block within a column, and into an adjacent column, shows the same accent-colored line as the top-level flow.

Done when: dragging a block to reorder it inside a column, or across columns, shows the drop-indicator line at the correct position, matching the top-level Document flow's existing behavior.

[brylie]

Set P2 (unset → P2): a cosmetic drag-feedback gap surfaced while implementing #239, matching the sibling maintenance/consolidation issues from that same release (#229, #241). No user-facing functional break (block reordering itself still works) and no other issue depends on it, so it doesn't warrant P0/P1.