PostCard: media-kind placeholder label in the reserved thumb box

[tannevaled]

What

PostCard reserved its thumbnail column only once a decoded image was present. A card whose post declares media but whose image has not yet landed drew a blank SurfaceAlt box. The reader's historical feed card drew the media kind (image/video) centred in that box while loading — so the toolkit migration read as "thumbnails not loading".

Change

• New field ThumbPlaceholder string (e.g. "image", "video").
• A non-empty placeholder reserves the thumbnail column even while Thumbnail is nil, so the card does not reflow (grow a column) when the image finally lands.
• postThumb draws the label centred in dim ink on the muted ground when no image is present, reproducing the old loading card.
• Column geometry now keys off reservesThumb() (image or placeholder) instead of hasThumb() (image only).
• The label is skipped when it has no font or would not fit the box.

Tests / gates

• New tests cover: placeholder reserves the column + no reflow when the image lands; label drawn in dim ink; and the three no-op branches (no font, too wide, too tall).
• 100% statement coverage on the package; gofmt clean; go vet clean.

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