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/parser.go

@@ -43,6 +43,17 @@ type Warner interface {
 	Warnings() []string
 }
 
+// Namer is an optional interface for parsers whose bank names its downloads
+// unhelpfully. StatementName reads the statement at path and returns the name,
+// without extension, it should be stored under — the upload's extension is
+// kept, lowercased — or "" when the statement does not say, in which case it
+// keeps the name it came with. Only an upload uses
+// it; files already in a folder are never renamed, since the index records
+// them by path.
+type Namer interface {
+	StatementName(path string) (string, error)
+}
+
 // Factory builds a parser from an account's config, validating it up front so
 // a bad account.toml fails before any file is read.
 type Factory func(acc *config.Account) (Parser, error)