[import] Warn before CSV update creates unmatched entities

[frankrousseau]

Problems

• When updating existing shots/assets/edits from a CSV, lines whose names don't match any existing entity (typically leading zeros stripped by the spreadsheet, 001 becoming 1) were silently created, polluting the production; the only signal was a subtle row-color difference in the preview table (fixes When we update data with a CSV file, if the number of digits is wrong for the shots name, it will create unwanted shots #771)
• The Shots and Edits pages built the existing-entity lookup from the filtered display list, so entities hidden by an active search were wrongly considered new

Solutions

• In the import preview modal, when "Update existing data" is checked and at least one line matches no existing entity, show a warning box listing the entries that will be created (with a count) and disable the confirm button until an explicit "I confirm that I want to create these new entries" checkbox is ticked
• Extract the created-entries computation to csv.getNewEntityNames() in src/lib/csv.js and cover it with unit tests
• Build filteredShots / filteredEdits from the full entity caches instead of the filtered display lists, mirroring the Assets fix for Importing CSV only checks loaded assets if already imported #927