Name NLB uploads, delete statements, and stop importing on upload

Four changes to statement handling in the web app, made together and
touching the same upload and statements-list code.

Name NLB uploads by statement date. parser.Namer is an optional
interface, like Warner, through which a parser names its statements;
nlb reads the "Datum izpiska" from the izpisek header and names it
izpisek_YYYY_MM_DD, lowercase, extension included -- ported from the
rename_izpiski.py it replaces. Uploads are staged as dotfiles, invisible
to import, so the parser can read them; two downloads of one statement
then meet under one name and the second is recognised as already there,
while a different statement of the same date is numbered _2 as the
script did. Only uploads are named: source_files records statements by
path, so renaming a file already in a folder would orphan its rows.

Delete a statement from the statements list. The file is removed from
disk for good -- the page says so before it asks -- and
store.ForgetSourceFile drops its transactions and their transfer rows.
A row two overlapping statements share is stored once, under the file
imported first, so it goes too; the account's other statements forget
their checksums and show as changed until the next Import re-reads them
and restores it. A file already gone from disk can be forgotten.

Upload and delete no longer import. Importing stays the user's call,
made with the Import button, so a batch can be put together and looked
over first. Delete still re-pairs transfers, which reads no statement.

Show rows and new rows per statement. The list read "0" for a file
whose rows an earlier, overlapping statement already held, which looked
like a file that failed to parse. source_files now records how many
transactions each statement holds, and the list reads "3 rows · 0 new".

This adds a column the code reads, so an index built by an earlier
version fails with "no such column: s.rows": delete index.db and import
again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/parser/nlb_test.go

@@ -112,3 +112,15 @@ func TestParseNLBRejectsMalformedLine(t *testing.T) {
 		t.Error("expected an error for a transaction line with no amount")
 	}
 }
+
+// An izpisek is named after its statement date, as rename_izpiski.py did.
+func TestNLBStatementName(t *testing.T) {
+	header := "                              IZPISEK 001/2026\n" +
+		"   Datum izpiska    31.01.2026          Stran 1\n"
+	if got := nlbStatementName(header + nlbPage); got != "izpisek_2026_01_31" {
+		t.Errorf("name = %q, want izpisek_2026_01_31", got)
+	}
+	if got := nlbStatementName(nlbPage); got != "" {
+		t.Errorf("name without a statement date = %q, want none", got)
+	}
+}