chore: pivot data layer to Nostr protocol (Note foundation)

[samarthsinh2660]

What changed and why

This PR establishes the Note data layer foundation for the Nostr protocol
and fixes a critical build toolchain incompatibility.

---

Deleted

• PostModel, PostEntity, PostRepository, PostUseCase
• CommentModel, CommentEntity, CommentRepository, CommentUseCase
• PostInput, CommentInput

In Nostr, everything is a Kind 1 Note. A "post" is a note with no parent.
A "comment" is a note with rootEventId set. No separate models needed —
this removes the redundancy.

---

Added

• lib/core/enum/note_type.dart — text | image | link | reference
• lib/data/models/note_model.dart — Isar collection for Kind 1 notes
• lib/data/models/note_model.g.dart — generated Isar schema
• lib/domain/entities/note/note_entity.dart — freezed domain entity
• lib/domain/entities/note/note_entity.freezed.dart — generated
• lib/domain/repositories/note_repository.dart — abstract interface
• lib/data/repositories/note_repository_impl.dart — Isar implementation
• CLAUDE.md — architecture context file (module map, layer rules, DO/NEVER
  list)

---

Modified

pubspec.yaml — Three dependency changes:

1. isar → isar_community 3.3.2
   Isar 3.x is unmaintained and incompatible with Dart 3.11 — build_runner
   crashes. The community fork is a drop-in replacement with the same API.

2. freezed → ^3.0.0, freezed_annotation → ^3.0.0
   Required by isar_community_generator (both need build ^4.x).

3. build_runner → ^2.13.0
   Older versions crash on Dart 3.11 due to a missing SDK snapshot file.

All *.freezed.dart and *.g.dart diffs — Auto-regenerated by
build_runner after the freezed 2.x → 3.x upgrade. Logic is unchanged,
only the generated boilerplate format changed.

local_datasource.dart — Added NoteModelSchema to Isar.open().

.gitignore — Added docs/ and uniun/ (local files, not for VCS).

---

What is NOT changed

• All existing community, user, and feed UI — untouched
• All existing BLoC logic — untouched
• App still compiles and runs as before