#323: PID + Source URL columns in samples table and CSV export

[rdhyee]

What this does

Implements @akthom's #323:

1. PID column in the samples table (was CSV-only) — placed after Label, ellipsized via CSS (ark:/28722/… pids are long) with the full pid on hover, and always complete in the CSV.
2. Source URL column — a dedicated external link (n2t.net resolver via the existing unit-tested sourceUrl() helper) to each sample's landing page at its source repository, in both the table and the CSV. In the CSV it's appended as the last column so Make it possible to download CSV of Samples Table #312-era consumers keep their column positions.

Design notes:

• Does not reopen Explorer: unify click semantics — click opens detail card, external link is one-hop-away #226 (which removed the label cell's href): the external link lives in its own honestly-labeled cell, and activateRow now ignores clicks originating inside an <a>, so a link click opens the record without also selecting/flying the row.
• Row-specific aria-label on each link (Codex review) so screen readers announce which record each identical "record ↗" link opens.

Codex review (5.6)

approve after one focused browser smoke test … It reuses the existing tested sourceUrl(); attribute values are escaped; CSV ordering remains backward-compatible; anchor clicks no longer trigger row selection; rel="noopener noreferrer" is correct. I found no code blocker.

Smoke test performed (fork preview, fresh isolated browser context)

• Headers render …|PID|…|Source URL; pid cell ellipsizes with full pid in title; link href=https://n2t.net/<pid>, target=_blank, rel=noopener noreferrer ✅
• Link click does NOT select/fly the row; normal cell click still does (regression check) ✅
• Link is keyboard-focusable ✅
• Narrow viewport (480px): table scrolls inside .table-scroll, body does not overflow (fresh load; matches prod behavior) ✅
• 0 console errors; 48/48 unit tests ✅

Closes #323.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AjnkWb4HpuLeDbYfzmY3X9

[rdhyee]

Codex 5.6 verdict (independent review): "approve after one focused browser smoke test… reuses the existing tested sourceUrl(); attribute values are escaped; CSV ordering remains backward-compatible; anchor clicks no longer trigger row selection; rel=noopener noreferrer is correct. I found no code blocker." The requested smoke test was performed (see PR body: link-vs-row click, keyboard focus, narrow viewport, fresh context) and its aria-label suggestion applied in 9ed195c.

— rbotyee 🤖