docs: add descriptions of the id and accountId

Author: wmontwe

core/architecture/api/README.md

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+# Core Architecture
+
+High‑level primitives shared across modules in this project.
+
+## ID
+
+Small, cross‑module primitives for strongly‑typed identifiers.
+
+### Core concepts
+
+- Principles:
+  - Keep IDs opaque and strongly typed (no raw strings at call sites).
+  - Centralize generation/parsing via factories to ensure consistency.
+  - Keep the core generic; domain modules extend via typealiases and small factories.
+- Building blocks:
+  - `Id<T>`: a tiny value type wrapping a UUID (kotlin.uuid.Uuid).
+  - `IdFactory<T>`: contract for creating/parsing typed IDs.
+  - `BaseIdFactory<T>`: abstract UUID‑based implementation of `IdFactory<T>` for standardized creation and parsing.
+
+Implement custom factories if you need non‑UUID schemes; otherwise prefer BaseIdFactory.
+
+### Usage
+
+Create a typed ID and parse from storage:
+
+```kotlin
+// Domain type
+data class Project(val id: ProjectId)
+
+// Typed alias
+typealias ProjectId = Id<Project>
+
+// Factory
+object ProjectIdFactory : BaseIdFactory<Project>()
+
+// Create new ID
+val id: ProjectId = ProjectIdFactory.create()
+
+// Persist/restore
+val raw: String = id.asRaw()
+val parsed: ProjectId = ProjectIdFactory.of(raw)
+```
+

feature/account/api/README.md

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+# Account API (feature/account/api)
+
+A small, feature‑agnostic API for representing accounts by identity only. It provides:
+
+- Strongly‑typed account identifiers (`AccountId`)
+- A minimal Account interface (identity only)
+- A unified account sentinel for aggregated views
+- Profile types for UI (`AccountProfile`) and an abstraction to fetch/update them (AccountProfileRepository)
+
+This module intentionally avoids feature‑specific fields (like email addresses). Mail, calendar, sync, etc. should
+attach their own capability models keyed by `AccountId`.
+
+## Core concepts
+
+- `Account`: Marker interface for an account, identified solely by `AccountId`.
+- `AccountId`: Typealias of `Id<Account>` (UUID‑backed inline value class). Prefer this over raw strings.
+- `AccountIdFactory`: Utility for creating/parsing `AccountId` values.
+- `UnifiedAccountId`: Reserved `AccountId` (UUID nil) used to represent the virtual “Unified” scope across accounts.
+  - `AccountId.isUnified`: Shorthand check for unified account id.
+  - `AccountId.requireReal()`: Throws if called with the unified ID. Use in repositories/mutation paths.
+- `AccountProfile`: Display/profile data for UI (name, color, avatar) keyed by AccountId.
+- `AccountProfileRepository`: Abstraction to read/update profiles.
+
+## Usage
+
+Create a new `AccountId` or parse an existing one:
+
+```kotlin
+val id = AccountIdFactory.create()            // new random AccountId
+val parsed = AccountIdFactory.of(rawString)   // parse from persisted value
+```
+
+Detect and guard against the unified account in write paths:
+
+```kotlin
+fun AccountId.requireReal(): AccountId // throws IllegalStateException for unified
+
+if (id.isUnified) {
+    // route to unified UI/aggregation services instead of repositories
+}
+```
+
+Fetch/update an account profile:
+
+```kotlin
+val profiles: Flow<AccountProfile?> = repo.getById(id)
+repo.update(AccountProfile(id, name = "Alice", color = 0xFFAA66, avatar = AccountAvatar.Monogram(value = "A")))
+```
+
+## Design guidelines
+
+- Keep Account minimal (identity only). Do not add mail/calendar/sync fields here.
+- Feature modules should define their own models keyed by `AccountId`.
+- Do not persist data for `UnifiedAccountId`. Compute unified profiles/labels in UI where needed.
+- Prefer strong types (`AccountId`) over raw strings for safety and consistency.
+
+## Related types
+
+- `Id<T>`: Generic UUID‑backed identifier (core/architecture/api)
+