Add source descriptions for manual imports

[Ayobami-00]

Summary

Fixes #37.

• Adds an optional source description sheet after manual image selection from the Today FAB and empty state.
• Stores the description locally on the source metadata and includes it in the queued extract_source job input.
• Threads the description into local AI prompts as import context while preserving source-evidence grounding.
• Treats sheet dismissal as cancel, while the explicit Skip action imports without a description.

Validation

• flutter analyze
• flutter test
• Targeted simulator/E2E check, if relevant

Targeted checks:

• flutter test test/features/source_ingestion/presentation/source_ingestion_cubit_test.dart test/features/source_ingestion/domain/source_ingestion_use_cases_test.dart test/features/ai_processing/domain/ai_job_queue_test.dart test/features/today/presentation/today_screen_test.dart
• flutter test integration_test/beta_feedback_icon_e2e_test.dart attempted; blocked locally because Xcode could not find the booted simulator destination.
• flutter test integration_test/beta_feedback_icon_e2e_test.dart -d macos attempted fallback; blocked because no macOS desktop project is configured.

Sanitized validation note: artifacts/issue_37_manual_import_description/validation.md (ignored).
Private demo proof uploaded for PR 38 attempt 2.

[greptile-apps]

Greptile Summary

This PR adds an optional source-description bottom sheet that appears after a user manually picks an image from the Today FAB or empty state. The description is stored in the source's metadataJson, forwarded through QueueSourceProcessingParams into the AI job's inputJson, and injected into both the vision-extraction and intention AI prompts as grounding context.

• Cubit orchestration: SourceIngestionCubit.importImage now owns the pick-then-describe sequence; a null return from the requestDescription callback (dismiss gesture) correctly aborts the import, while an empty string (Skip) proceeds without storing a description.
• Params promotion: NoParams replaced by ImportImageSourceParams carrying an optional pickedImage and sourceDescription; QueueSourceProcessingParams gains the same optional field, writing it into inputJson only when present.
• New tests: A dedicated cubit test file covers the dismiss-cancels and skip-imports paths; use-case and AI-job-queue tests verify trimming, metadata storage, and prompt injection end-to-end.

Confidence Score: 5/5

Safe to merge; the description field flows cleanly through all layers and dismiss/skip/save paths are covered by targeted tests.

The pick → describe → import sequence is correctly sequenced in the cubit, with explicit null-return semantics distinguishing cancel from skip. The description value is normalised before storage and before job queuing, keeping empty-string edge cases handled consistently. New unit and integration tests exercise all three user paths (dismiss, skip, save with text).

No files require special attention. The previously noted concerns about _normalizedSourceDescription duplication and repeated jsonDecode in _metadataString remain open but were flagged in an earlier review pass.

Important Files Changed

Filename	Overview
lib/features/source_ingestion/presentation/logic/source_ingestion_cubit.dart	Cubit now orchestrates pick → description sheet → import; dismiss (null) correctly cancels, skip ('') continues without description; logic paths are clear and well-guarded.
lib/features/today/presentation/screens/today_screen.dart	Adds _ManualImageDescriptionSheet with correct keyboard-avoidance animation, proper controller disposal, and context.mounted guard before showing the sheet.
lib/features/source_ingestion/domain/use_cases/import_image_source.dart	Params updated from NoParams to ImportImageSourceParams; pre-picked image consumed via pickedImage ?? pickImage() fallback; description normalised and written to metadata JSON.
lib/features/source_ingestion/domain/use_cases/queue_source_processing.dart	sourceDescription threaded into job inputJson; _normalizedSourceDescription duplicated from ImportImageSource (flagged in prior review); no logic errors.
lib/features/source_ingestion/domain/entities/source_item_entity.dart	sourceDescription getter added via _metadataString which re-parses metadataJson on every call (flagged in prior review); returns null cleanly on missing/empty/invalid JSON.
lib/core/ai/prompts/cactus_prompt_templates.dart	Description injected into both vision extraction and intention prompts via _sourceDescriptionPromptBlock; null-guarded; instructions preserve source-evidence grounding.
test/features/source_ingestion/presentation/source_ingestion_cubit_test.dart	New test file covering dismiss-cancels and skip-imports paths; real DB + file store wired up; assertions on metadata and queue params are thorough.
test/features/today/presentation/today_screen_test.dart	E2E test replaced with a data-seeding approach that skips the FAB → picker → description-sheet UI flow; verifies description propagates into both AI prompts.

_{Reviews (2): Last reviewed commit: "Cancel manual import on description dism..." | Re-trigger Greptile}