feat: pin any columns with an anchor and keep merged block label in view

[daggerok]

What changed

• Anchor button in every column header pins the column: it stays in place and sticks to the left edge on horizontal scroll. Several columns can be pinned, they stack in column order
• Tab view: pins are per file (keyed by file name). Merge view: one shared set, since all blocks share columns. Both persist in settings (pinnedColumns, pinnedColumnsByFile) and pass through sanitizeSettings
• 1st Col = Sticky toggle removed: nothing is pinned by default
• Merged Block label sticks to the right edge of the viewport, visible without scrolling to the end

How it was checked

• Built with Parcel and drove it in headless Chromium (Playwright) with two Fidelity positions CSVs:
  • no pinned columns by default
  • file 1: pinned Col 4 and Col 9, after scrolling right both sit at the left edge (x 17 and 183, Col 4 width 166), body cells aligned with headers
  • file 5: starts with no pins, pinned Col 2; switching back to file 1 still shows 4 and 9; both survive a reload
  • merge view: no pins of its own, pinned anchors 180px apart after scroll, label right edge equals the container right edge before and after scroll
• bun build src/main.tsx --outfile=/dev/null, git diff --check, tc/check.sh shows no new non-JSX errors