fix(notes): handle nullish and non-iterable placeholders in isRegenerableNoteTitle

[hsusul]

Fixes #1815

Problem

In src/helpers/regenerableNoteTitle.js:
isRegenerableNoteTitle defaulted placeholders = [], which only triggers when placeholders === undefined. If passed null or a non-iterable value, iterating with for (const p of placeholders) threw TypeError: placeholders is not iterable.

Solution

• Fall back to [] if placeholders is null or not iterable before iterating.
• Added unit tests in test/helpers/regenerableNoteTitle.test.js.

Verification

• node --test test/helpers/regenerableNoteTitle.test.js (passes, 6/6 tests)
• npm run typecheck (passes, 0 errors)
• npm run lint (passes, 0 errors)
• npm run i18n:check (passes)
• npm run build:renderer (passes)
• git diff --check (clean)

[Chadpiha]

Thanks @hsusul! Closing together with #1815.

This helper has a single caller (PersonalNotesView.tsx), which always passes a literal inline array of three t(...) results — and react-i18next's t() always returns a string — so placeholders is a string array by construction on every call. The one parameter that genuinely crosses a trust boundary here is title (it comes from the DB and can be null), and the helper already guards that with typeof title === "string". Our convention is to draw the line exactly there: validate what crosses a boundary, trust what internal code guarantees.

The tests themselves were well done and I ran them green — this is purely a scope-of-validation call, not a quality one. Thanks for the contribution, and keep 'em coming!