feat(miniapp): clarify balance breakdown

docs/specs/HOUSEBOT-078-miniapp-balance-breakdown.md

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+# HOUSEBOT-078 Mini App Balance Breakdown
+
+## Goal
+
+Make the mini app read like a real household statement instead of a generic dashboard shell.
+
+## Scope
+
+- highlight the current member's own balance first
+- show base due (`rent + utilities`) separately from the shared-purchase adjustment and final due
+- keep full-household balance visibility below the personal summary
+- split ledger presentation into shared purchases and utility bills
+- avoid float math in UI money calculations
+
+## Notes
+
+- no settlement logic changes in this slice
+- use existing dashboard API data where possible
+- prefer exact bigint formatting helpers over `number` math in the client