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/queries.ts

@@ -22,6 +22,9 @@ export interface TransactionRow {
   category: string;
   row_index: number;
   created_at: string;
+  // loan repayment split — set only when the lender itemises it (migration 0014)
+  principal_amount: number | null;
+  interest_amount: number | null;
   // override fields
   category_override: string | null;
   merchant_override: string | null;
@@ -64,6 +67,12 @@ export interface StatementRow {
   credit_limit: number | null;
   currency: string;
   statement_type: string | null;
+  // Loan statements only (see migration 0014)
+  interest_rate: number | null;
+  scheduled_repayment: number | null;
+  repayment_frequency: string | null;
+  redraw_available: number | null;
+  loan_term_months: number | null;
   tier_used: string | null;
   owner_id: number;
   owner_name: string;