feat(loans): principal/interest split so repayments stop distorting spend

A loan repayment is not an expense. A $3,000 mortgage repayment is roughly
$1,200 of principal (equity — a balance-sheet move) and $1,800 of interest (the
only part that is genuinely spend).

Migration 0014:
- transactions.principal_amount / interest_amount, populated only when the lender
  itemises the split on the repayment row
- statements.interest_rate, scheduled_repayment, repayment_frequency,
  redraw_available, loan_term_months
- normalize_repayment_frequency() + trigger, so "Fortnightly", "Bi-Weekly" and
  "Every 2 weeks" all land on 'fortnightly'

Two statement shapes are handled. Where the loan statement lists repayments and
"Interest Charged" as separate rows (the common Australian case), transaction_type
already does the work. Where a lender itemises the split on the repayment row,
that row is typed 'payment' and would be skipped entirely — losing the interest.
New SPEND_ROWS / SPEND_BASE fragments in analytics-sql.ts count such rows at
interest_amount instead of amount.

Adds the loan_interest category (+ colour, and the missing fees colour).

Verified against the live DB with a synthetic ANZ home loan statement: a $3,000
itemised repayment plus a $10 service fee moved April spend by exactly $1,810,
with the $1,200 principal excluded and still retained on the row. Test data
removed and the figure confirmed back at its original value.

src/lib/analytics-sql.ts

@@ -21,6 +21,24 @@ export const STATEMENTS_JOIN = `LEFT JOIN statements s ON s.id = t.statement_id`
 /** Transaction types that represent money going out. */
 export const SPEND_TYPES = `('debit', 'fee', 'interest')`;
 
+/**
+ * Rows that count towards spend.
+ *
+ * The `interest_amount IS NOT NULL` arm is for loan repayments: when a lender
+ * itemises principal and interest on a single repayment row, that row is usually
+ * typed 'payment' (money reducing the loan balance) and would otherwise be
+ * skipped — but its interest portion is real spend.
+ */
+export const SPEND_ROWS = `(t.transaction_type IN ('debit', 'fee', 'interest') OR t.interest_amount IS NOT NULL)`;
+
+/**
+ * The amount of a row that counts as spend, before split adjustment.
+ *
+ * For an itemised loan repayment only the interest portion is an expense; the
+ * principal builds equity and is a balance-sheet move, not spending.
+ */
+export const SPEND_BASE = `CASE WHEN t.interest_amount IS NOT NULL THEN t.interest_amount ELSE COALESCE(t.amount_aud, t.amount) END`;
+
 /** Effective category, honouring overrides. Never NULL. */
 export const EFFECTIVE_CATEGORY = `COALESCE(o.category_override, t.category, 'other')`;