feat: expense tracker Python parity (Wave 10)

New `expenses_handlers.py`:
- parse_expense(phrase) -> (amount, category) | None  — pure fn, fully
  unit-tested. Handles "потратил 500 на еду", comma decimals "12,50",
  EN "spent 12.5 on coffee", default category "разное".
- do_expenses_log: append entry to expenses.json (atomic write-then-rename).
- do_expenses_today: sum today's entries (local-day boundary).
- do_expenses_week: rolling 7-day total + count.
- do_expenses_breakdown: per-category, top-5 + tail collapse.

extensions.py / main.py / commands.yaml: 4 new dispatch entries.
.gitignore: + expenses.json.

tests/test_expenses_handlers.py: 8 unit tests covering parse, log,
today/week math, breakdown ordering + tail collapse.

Tests: 96 → 104 (+8). Total Python coverage now mirrors Rust pack
behaviour for personal finance.

.gitignore

@@ -17,6 +17,7 @@ llm_backend.txt
 llm_history.json
 active_profile.txt
 time_tracker.json
+expenses.json
 
 # C extensions
 *.so