table_default card_grid_class, MediaGallery hide-Add-at-limit, draggable_list responsive cols

[timujinne]

Summary

Three additive, backwards-compatible changes to core components, driven by the Document Creator image-picker work:

• table_default card_grid_class attr — lets consumers override the card-view grid density (column count, gaps) without touching the per-view-mode display utility the component sets itself. Default keeps the previous 1/2/3/4-col layout. The value must NOT carry a display class (grid/hidden) so controlled table mode keeps emitting only hidden — preserving the source-order-independent fix from the post-MediaGallery: add max_count attr + disable Add button at limit #556 follow-up.
• MediaGallery hides the Add tile at the selection limit — when the selection reaches max_count (or 1 in :single mode) the Add tile is omitted entirely instead of rendered as a disabled, greyed-out tile.
• draggable_list cols accepts a responsive class string — cols now takes either an integer (1..6, mapped to a static grid-cols-N as before) or a string of Tailwind grid-column classes used verbatim, e.g. "grid-cols-4 lg:grid-cols-6 2xl:grid-cols-8", so consumers can render a responsive thumbnail grid. MediaGallery plumbs the value straight through.

All three preserve existing behavior for current call sites (defaults unchanged; integer cols callers unaffected).

Test plan

• Existing <.table_default> / <.draggable_list> / <.MediaGallery> usages render unchanged
• card_grid_class overrides density without breaking controlled table-mode hiding
• MediaGallery Add tile disappears at the limit and returns when an item is removed
• draggable_list with a responsive cols string renders a responsive grid (verify the larger-breakpoint column count actually wins — class must cascade after smaller breakpoints)

[timujinne]

PR #561 Review

Verdict: PASS with one MEDIUM bug to fix

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Stage 1: Spec Compliance — PASS

All three commits implement what was requested and are backwards compatible:

1. table_default card_grid_class — default matches previous hardcoded value; display utility still set per-branch at table_default.ex:334-336; doc warns against including display classes. No existing call-sites broken.

2. MediaGallery hide Add tile — :if guard correctly combines not @readonly + not selection_at_limit?(...) at media_gallery.html.heex:75; disabled-state classes and disabled attr removed; doc updated.

3. draggable_list cols integer|string — binary guard clause correctly placed before integer clauses at draggable_list.ex:168; default remains 4; doc mentions Tailwind literal scanning requirement. MediaGallery moduledoc updated.

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Stage 2: Code Quality

BUG - MEDIUM: Empty grid cell when Add tile is hidden

media_gallery.html.heex:71-85 + draggable_list.ex:156-160

When selection_at_limit? returns true, the :if on the <button> (line 75) prevents the button from rendering, but the <:add_button> slot itself is still declared. In draggable_list.ex:156, the check @add_button != [] evaluates to true because Phoenix slots are lists of slot entries — the list is non-empty regardless of the inner :if. The <div class="sortable-ignore"> wrapper renders as an empty div, creating a visible empty grid cell.

Fix: move the :if guard from <button> to the <:add_button> slot:

<:add_button :if={not @readonly and not selection_at_limit?(@selected, @mode, @max_count)}>
  <button type="button" class="..." ...>
    ...
  </button>
</:add_button>

This excludes the slot entry entirely, so @add_button == [] and the wrapper div is not emitted.

NITPICK: cols attr type :any is broader than needed

draggable_list.ex:87 — accepts atoms, lists, maps, etc. Only integers and strings are valid. No crash risk (catch-all falls back to grid-cols-4), and doc already describes expected types. No action needed.

NITPICK: Tailwind breakpoint cascade

draggable_list.ex:90 — doc doesn't mention ascending breakpoint order requirement, but the example already shows correct ordering and this is standard Tailwind knowledge. No action needed.

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Quality Summary: 0 critical, 1 medium, 0 minor, 2 nitpick

Overall: PASS after fixing the empty grid cell — move :if from <button> to <:add_button> slot in media_gallery.html.heex.