[callumalpass]

Summary

Refs #2436 and #909. Thanks to @RumiaKitinari for the request.

• Add Use default projects only when no project is specified under Task Properties → Projects, disabled by default for backwards compatibility.
• When enabled, NLP project input and the creation form's project picker replace only preloaded default projects. Explicit selections, Bases prepopulation, and parent/subtask assignments are preserved, including intentional assignments that happen to name a default project.
• Apply the same policy to instant conversion while preserving parent-note/heading associations and the existing conversion-defaults switch.
• Translate the new setting in all supported interface languages and document the behavior. Removing an explicit project leaves the selection empty rather than restoring defaults.

Validation

• Full Jest suite: 744 suites / 5,673 tests passed; 69 suites / 1,000 tests skipped by the existing suite, 4 todo. Includes 16 new unit cases for fallback policy and provenance.
• Isolated native Obsidian regression: 3 passed (e2e/issues/issue-2436-default-project-fallback.spec.ts), covering legacy additive mode, explicit-project override, and no-project fallback. Used separate profile/vault/debug port with instance reuse disabled; verified vault over CDP.
• npm run lint (strict TypeScript, review-types, CSS, architecture): passed.
• npm run typecheck: passed.
• Strict translation/source checks (run by ordinary build/Jest preparation) and npm run test:translation-contracts: passed.
• Production build and isolated-vault build:test: passed.
• git diff --check: passed.

Recorded desk test

Isolated desk gcp/tn-2436, session 20261029T132653Z (desk stopped after testing; recording summary saved).

• 00:27:33: baseline NLP +[[Personal Projects]] with Homework default saved both projects, confirmed in YAML.
• 00:31:59: fixed build, fallback enabled: saved only Personal Projects.
• 00:32:08: control without explicit project: saved Homework.
• 00:35:12: inspected settings in the native light-theme popout; toggled the new option and verified persisted setting.
• 00:35:58: project picker replaced Homework with Personal Projects; verified saved YAML.
• No Obsidian console warnings during these checks.

The issue remains open for review. No release publication or merge performed.