* feat(accounts): add Gains / ROI chart view for investment accounts
Adds a fourth chart view to the account details page showing the
historical unrealized gains series (market value - cost basis per
holding, summed daily with LOCF and FX conversion), following the
same ChartSeriesBuilder/Series pipeline as the existing views.
Holdings without a usable cost basis (nil, or unlocked zero from
providers) contribute zero gain, matching Holding#avg_cost semantics.
* fix(accounts): carry cost basis forward over gap-filled holdings in gains series
Gap-filled holding rows (weekends, price-history gaps) are persisted
without cost_basis even though the position and basis are unchanged,
which zeroed the gains series on those dates. Look up the basis from
the latest snapshot that has a usable one instead of reading it from
the current row, so already-persisted gap-filled rows are handled too.
* docs(accounts): add method docstrings for gains chart view code
Satisfies the pre-merge docstring coverage check on methods added or
touched by the Gains / ROI feature.
* fix(accounts): sign converted amount like main indicator in gains view
Extracts the gains sign-prefix logic into a shared signed_format helper
so the family-currency converted amount shown on foreign-currency
accounts matches the main indicator (+€79.53 / +$85.00), and adds
component tests covering the positive, negative, non-gains and
foreign-currency formatting paths.
* test(accounts): cover FX conversion path in gains series
The gains_series tests only used USD holdings against a USD target,
leaving the exchange_rates LATERAL join untested. Adds a case with EUR
holdings converted to USD, including LOCF rate carry-forward.
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Co-authored-by: Antoine GUYON <agy@ibanfirst.com>
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