Scope: Surface "add a column" as a discoverable, intuitive button at the end of the table — adding a column is currently buried inside the "Templates and columns" modal toolbar button, where most users never look.

What's wanted:

• A full-height vertical button at the end of the visible columns (after the last data column, in the spot where a *future* column would land), so the affordance literally sits at the position of the future column.
• Click it → small popover offering the most-relevant column choices directly:
  1. Quick-add list: a short list of currently-hidden built-in columns that most users want to surface, each clickable to enable in one click. Suggested seed list: Source, Original filename, Lens, Focal length, Date taken, Camera location, Object location, Location of creation (P1071) (filtered to those not already visible).
  2. Add Wikidata property as column: inline search input mirroring the existing autocomplete in ColumnMenu (currently dead code — see below), powered by window.KNOWN_PROPERTIES and (live) loadVocabPropertiesByQuery. Picking a result calls the same addCustomProp path the Templates-and-columns modal uses today.
  3. Add custom wikitext-template column: a link/button that dispatches into the existing CustomColumnCreator flow — easiest route is to open the Templates-and-columns modal at the Columns tab with the custom-column form auto-expanded.
  4. More options → link to open the full Templates-and-columns modal (so power users still have the full surface available).

Where in the code:

• src/table.jsx:711–843 renders the table. The header row already terminates with <div className="tbl__th tbl__th--actions" /> (table.jsx:802) — that's where the new column button anchors.
• src/table.jsx:497–503 builds gridTemplate. The new button cell needs to be added there (or rendered as a sticky/separate element, see UX note below) and in the head row + each Row.
• src/table.jsx:455–493 already exposes addCustomProp(p) and addTemplateColumn(label, template) and the surrounding setColState mechanics — the new button just dispatches into them.
• src/table.jsx:4635–4738 defines ColumnMenu (Wikidata-property autocomplete). It's currently dead code (rendered nowhere). The new popover can either reuse it or copy its property-search pattern.
• src/columns-modal.jsx:332–422 defines CustomColumnCreator — keep using it via the modal route.
• src/table.jsx:845–883 mounts the ColumnsModal driven by colMenuOpen (false | 'columns' | 'templates'). Easiest dispatch for "open columns modal" is setColMenuOpen('columns').

Acceptance criteria:

• A visible button labelled something like "+ Add column" appears at the right end of the visible-columns block in both the stash table and the history table, and inside Groups view's per-group mini-tables (or the toolbar above them — see open question below).
• The button visually reads as a column-shaped affordance (full row-block height, vertical orientation acceptable; doesn't collide with the existing per-column resize handle on the rightmost column).
• Clicking it opens a popover anchored to the button.
• The popover contains: (a) the quick-add list of currently-hidden default-most-relevant columns, (b) Wikidata property search, (c) "Custom wikitext-template column" entry-point, (d) "More options →" link to the full Templates-and-columns modal.
• All add-paths persist correctly (verified by reload — the colState save flow already covers visible, order, customProps).
• Existing "Templates and columns" toolbar button keeps working unchanged.
• npm run build passes (the undefined-identifier scanner runs first).
• Manual test in npm run dev: open dev server, click the new button, exercise each of the four affordances, reload, confirm columns persist.

Notes / non-goals:

• The columns-modal's "Add custom wikitext-template column" UX stays the source of truth for the inline form — the button just needs to surface it earlier in the discovery path. No need to reimplement a parallel form inline in the popover (premature; the existing one is good).
• Dead-code ColumnMenu in table.jsx:4635–4738 can either be wired into this popover (if its UX matches) or left alone for now — out of scope to delete it.
• Mobile: the table is already horizontally scrollable on small screens, so the button-at-end-of-columns position works for mobile too (you scroll right to reach it). No special mobile pattern needed.

Refs:

• Template / columns infrastructure: src/columns-modal.jsx, src/wikitext-templates.js
• Vocab pools: src/vocabulary.js (window.KNOWN_PROPERTIES)
• Persistence: localStorage key stashhub.columns.v10, user-store Preferences.json (customProps)