Offer negative clone offsets, and validate the interval before using it

Shifting backwards already worked -- the offset is added as a Postgres
interval, and '-90 days' is a perfectly good one -- but the suggestions only
listed forward shifts, so nothing said so. Pulling a plan back a quarter is
a normal thing to want.

A day-level shift is also the case that most needs the dim_period
derivation: 15 Mar 2027 less 90 days is 15 Dec 2026, which crosses from
2027/10 - Mar into 2027/07 - Dec. Copying the period columns across would
have labelled it March.

The offset is interpolated into the statement, so a typo surfaced as a
Postgres parse error from the middle of a CTE. It is parsed on its own
first, where the failure is cheap and can name the field.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

routes/operations.js

@@ -734,6 +734,21 @@ module.exports = function(pool) {
             const scaleFactor = (scale != null) ? parseFloat(scale) : 1.0;
             const dateOffset  = (date_offset || '0 days').trim() || '0 days';
 
+            // The offset is interpolated into the SQL as an interval literal, so a
+            // typo would surface as a Postgres parse error mid-statement. Ask
+            // Postgres to parse it on its own first, where the failure is cheap and
+            // can be reported against the field the user typed it into. Negative
+            // intervals are fine and useful -- '-90 days' pulls a plan back a
+            // quarter -- so this checks validity, not sign.
+            try {
+                await pool.query(`SELECT $1::interval`, [dateOffset]);
+            } catch {
+                return res.status(400).json({
+                    error: `"${dateOffset}" is not a valid interval. Try something like `
+                         + `"12 months", "-90 days" or "0 days".`
+                });
+            }
+
             // Cloning from a named segment is how a period with no baseline gets a
             // shape: pick the ledger line -- prior year, plan -- and copy its mix
             // forward. That means reading rows exclude_iters normally keeps

ui/src/components/OperationPanel.jsx

@@ -817,6 +817,8 @@ export default function OperationPanel({
                                <datalist id="pf-clone-offsets">
                                  <option value="12 months" />
                                  <option value="24 months" />
+                                 <option value="-90 days" />
+                                 <option value="-12 months" />
                                  <option value="0 days" />
                                </datalist>
                              </div>