Classify dividend payments by distribution type (capital gain, return of capital, etc.)

Today every cash distribution captured into accounts_dividend lands in one bucket. ETFs that distribute realized capital gains (common for SMICHA and other distributing CH/UK/US ETFs) get mixed in with regular dividend payments, so the dividend report and any tax export treat them identically. That is incorrect for UK / CH / US filers where capital-gain distributions are taxed differently from ordinary dividends.

Why now: 777 users hold something that looks like a distributing ETF in our data. Surfaced by ticket #145 where the annual ~1.5 CHF September SMICHA distribution is the giveaway pattern (consistently 2x the spring payments, once a year).

What we cannot do: rely on upstream data feeds. Neither FMP /stable/dividends nor EODHD /api/div/ return a type field on dividend payloads — they both lump everything into one amount per date. Real classification only exists on broker tax documents.

Proposed scope (MVP):
- Add a distribution_type enum on accounts_dividend and accounts_dividendcustom with choices: ordinary, capital_gain, return_of_capital, interest. Default 'ordinary' so all existing rows remain unchanged.
- Per-row dropdown on the dividend page so users can reclassify a distribution.
- Tax report respects the classification (capital_gain feeds the realized-gains line, return_of_capital reduces cost basis, ordinary stays in dividend income).

Phase 2 (later):
- Heuristic flag: if an ISIN has a recurring annual distribution notably larger than its other payments, suggest user review.
- Bulk reclassify by ISIN (one click marks every September payment for SMICHA as capital_gain).