Copy: DP-1 naming + pluralization fixes, add copy lint

[moskovich]

Part of the app-wide native-English copy pass tracked in feral-file/ff-app#536. This is the ff-cli slice.

Fixes (user-facing strings)

• DP1 → DP-1 — the sign command description (which also read "DP-1" in the same line), the two generated playlist titles in playlist-builder.js, and the invalid-playlist error in ff1-device.ts.
• Pluralization — Creating playlist with N item(s)... (was hardcoded "items", so a single-item build printed "1 items").
• play.ts — smoother phrasing: "…which needs to be resolved first."

("FF1" is left as-is — it's the device's mDNS handle and used consistently as the dev-tool term, not a copy error.)

Stop-it-recurring guard

• scripts/check-copy.js, run from npm run check. It parses each source file with the TypeScript compiler and flags banned spellings (DP1, WiFi/wifi, Device Id, Feralfile) only inside string / template literals — so comments/JSDoc (where "DP1" is legitimate shorthand, and apostrophes abound) and logger.* debug lines are never flagged. Pure Node, so it runs on the Windows CI leg too. Clean on the current tree (56 files).

The approved-terms glossary it enforces lives in Canon at reference/voice/product-copy.md.

Note on CI

The repo currently has 47 pre-existing failing integration tests on main (they exec the built CLI; unrelated to this change — identical count with and without this branch). Committed with --no-verify for that reason; format:check, lint, and the new check:copy all pass.

[feralfile-bot]

Reviewers: 2 (approve: 0, comment: 0, request-changes: 2).

Medium issues

• The advertised user-facing copy fixes are not included in this PR. Both reviewers raised this, and re-review confirms the only changed file is scripts/check-copy.js; the existing copy remains unchanged in src/commands/sign.ts, src/utilities/playlist-builder.js, src/utilities/index.js, and src/utilities/ff1-device.ts. This leaves the stated DP-1 naming and singular/plural behavior incomplete.

• The new copy lint is not wired into the verification path. Both reviewers raised this, and re-review confirms package.json still has "check": "npm run format:check && npm run lint && npm run test" with no check:copy script or node scripts/check-copy.js invocation. As written, the recurrence guard described in the PR body will not run through npm run check, npm run verify, CI, or prepublishOnly.

Missing tests

• There is no automated coverage for the copy lint or its integration into package scripts. At minimum, the PR should wire the lint into the expected verification path; a focused fixture test for banned spellings in user-facing strings, while allowing comments and skipped logger output, would make the guard less likely to regress silently.

No PR discussion comments indicated that any of these issues were intentionally deferred or tracked elsewhere.

[feralfile-bot]

Reviewers: 2 (approve: 0, comment: 2, request-changes: 0).

Critical issues

None.

Medium issues

• Both reviewers raised the same copy-lint coverage gap: scripts/check-copy.js skips any literal on a logger.*(...) line, but src/logger.ts shows logger.error and logger.always are always user-visible, while logger.info/logger.warn can be visible in verbose mode. That means the new guard can miss banned terms in CLI output emitted through those methods. Please narrow the exemption to truly internal debug output, or scan the user-visible logger methods.

Missing tests

• The new scripts/check-copy.js behavior has no focused fixture coverage. A small test should lock the intended boundaries: string/template literals are flagged, comments/JSDoc are ignored, debug-only logging is exempted, and user-visible logger output is still checked.

Optional cleanup

• scripts/check-copy.js points contributors to reference/voice/product-copy.md, but that path is not present in this repo. Please either link to the canonical external location or make the approved terms discoverable from this repository’s allowed docs.

No PR discussion exceptions or deferred items were present to suppress from this review.