fix(inventory+history): address post-merge CR findings on #792 #896

Follow-up from CodeRabbit's post-merge review on #792 (review posted after merge, so the findings can't be addressed inside #792).

PR #792 added the "Monthly Cost" column to two history tables and renamed/restructured the active-commitments table, but three skeleton-loader callsites still reference the pre-#792 column counts in code + comments. The skeletons therefore render with the wrong number of placeholder cells, so the shimmer is one cell short of the real table on first paint.

Findings (all from the post-merge CR review on #792)

• frontend/src/history.ts:217 — showSkeletonRows(listEl, 8, 11) should pass 12 (purchase-history table now renders 12 <th> per renderHistoryList lines 652-663: Status / Date / Provider / Service / Type / Region / Count / Term / Upfront Cost / Monthly Cost / Monthly Savings / Plan). Comment on line 213 also still says "(11)" and omits "Monthly Cost" from the column list.

• frontend/src/history.ts:224 — showSkeletonRows(queueEl, 3, 8) should pass 12 (approval queue table now renders 12 <th> per lines 948-959: Date / Account / Provider / Service / Count / Term / Payment / Monthly Cost / Upfront Cost / Monthly Savings / Created by / Actions). Comment on lines 218-220 is also out of sync (says "3 rows x 8 cols", lists 8 headers).

• frontend/src/inventory.ts:86 — Comment says "5 rows × 10 cols" but ACTIVE_COMMITMENTS_COLS = 11 (line 72) after feat(inventory+history): align Monthly Cost/Savings columns across both tables #792 added "Monthly savings". The runtime value is correct (the comment is the only thing out of sync), but the comment is misleading for future readers.

All three are quick-win comment/literal updates with no behavioural blast radius beyond the shimmer appearance during the first render of these tables.

Scope

• Comment + skeleton-call-site updates only — no test changes required (renderer behaviour unchanged; skeleton row count is purely cosmetic and not asserted in tests).
• No other files touched.

References

• PR feat(inventory+history): align Monthly Cost/Savings columns across both tables #792 (closes Active Commitments + Purchase History show different subsets of per-commitment columns (Inv 1.1) #788, relates to Main Header global filter not propagated to Inventory & Coverage subpages (QA 2.7+2.14, 2.8+2.15) #866)
• Post-merge CodeRabbit review on feat(inventory+history): align Monthly Cost/Savings columns across both tables #792 (2026-06-01)