Detection handed back a configuration it had never tested, and only ever
ran behind the magic-wand button. A source opened for the first time
therefore started on `defaultConfig` — `;`, `DD/MM/YYYY`, columns 0/1/2 —
plausible enough to import a whole file wrong rather than fail visibly.
`detectImportFormat` now REPLAYS what it just decided over every data row
of the file and returns the rate as a `DetectionScore`. The replay runs
`mapRow`, the same pure function `parseFilesInternal` runs at import time,
under the same column-level decimal arbitration: two mappers would be the
exact divergence this chantier removes — a score reading 100 % while the
import wrote different amounts. A test asserts the two agree row for row
on every fixture of the corpus.
The threshold is 90 %, and it only colours the banner. Below it the panel
warns and prints the detailed count ("132 of 150 rows read"); at or above
it the banner is neutral. Nothing is blocked, because a perfect score says
nothing about the SIGN of what was read — `all-positive` scores 100 %
while every credit imports as an expense — and the preview step (#329) is
the real net, traversed at every import.
Detection now also fires on its own, guarded on `!existing`: a source that
has never been configured. A source that HAS one is never re-detected, the
stored format wins. The condition is deliberately not `!restored` — a
stored format that fails to decode already reports its own error, and
detecting over it would replace that message with a silent guess.
The button and the automatic run share one `detectFormatForFile`, so the
button replays detection instead of running a second, drifting variant of
it. The score is cleared when its source changes, when the format is
edited by hand (compared through the codec, so a rename keeps it) and when
a template overwrites the format — a banner vouching for a configuration
nobody measured is the misinformation it exists to remove.
Finally, the sign-convention selector is hidden in debit/credit mode.
`mapRow` computes `credit - debit` on magnitudes there and never reads
`signConvention`, so the control changed nothing. Hidden, not reset: the
stored value is left untouched.
Tests: 1054 vitest (+20), build and cargo check green. No DB migration.
CHANGELOG and docs are centralized in link 10 of the stack per the plan.
Resolves#328
