feat: add sidebar sections and inline channel icons to menu bar dropdown

[slackstat]

Summary

• Sidebar sections: Menu bar dropdown now mirrors Slack's sidebar organization with proper section headers — Mentions at top, then user-created sections (e.g. "Team"), then virtual catch-all sections (Channels, External Connections, Direct Messages). Empty sections are hidden.
• Inline channel icons: Private channels show a lock icon, public channels show #, both rendered as SF Symbol icons via NSTextAttachment to avoid the image-column padding that NSMenuItem.image causes.
• Right-aligned timestamps: Relative time (e.g. "3m ago") is right-aligned using NSParagraphStyle tab stops and dimmed with secondaryLabelColor.
• New API endpoint: Added users.channelSections.list to fetch sidebar sections, with client.userBoot as fallback.
• Model additions: isPrivate and isExtShared fields on ConversationInfo and ConversationItem, with corresponding NameCache entries.
• Tests: 6 new tests (39 total, all passing) covering section decoding, virtual section assignment, and mentions-first grouping.

Changes

File	What changed
SlackModels.swift	Added isPrivate, isGroup, isExtShared to ConversationInfo; resilient channelSections decoding
NameCache.swift	Added channelPrivate and channelExtShared caches
SlackAPIClient.swift	Added fetchChannelSections() method; removed debug logging
StateStore.swift	Three-pass groupBySections() (mentions → explicit → virtual); resolveChannelName() returns (name, isPrivate, isExtShared) tuple
SlackStatApp.swift	Inline SF Symbol icons via NSTextAttachment; right-aligned time; section header styling
ModelsTests.swift	Tests for isPrivate, isExtShared, missing sections decoding
StateStoreTests.swift	Tests for virtual section assignment, mentions-first grouping

Test Plan

• 39/39 tests pass
• Release build compiles cleanly
• Verified visually in menu bar