Feature: Mark room and space as read (manual)

[mjkatgithub]

Planning

• Milestone: Phase 3
• Priority: P2
• Size: S
• Estimate: 2 SP

Goal

Manual Mark as read for a single room and for all channels under a space (Element-style), without opening each room.

Background

• Auto mark-read on open already exists (useRoomUnread, markRoomAsRead).
• Missing: explicit user action from sidebar (room ⋯ menu / space header menu).
• Space-level: mark all descendant channels (via m.space.child tree, same as unread rollup in spaceUnread.ts) as read.

Scope

• Room: "Mark as read" in RoomCategoryList ellipsis menu when room has unread.
• Space: "Mark space as read" in space header / space menu when space has unread badge.
• Reuse markRoomAsRead; add batch helper for space child room IDs (collectSpaceChildRoomIds).
• Update unread indicators immediately after success.
• i18n EN/DE.

Branch

feature/mark-as-read

Acceptance Criteria

• Room with unread: menu action clears room unread (and mention) indicator.
• Space with unread rollup: action clears unread for all channels in that space subtree.
• Active room/thread behavior unchanged (still auto-read on view).
• No mark-read when already read (action hidden or disabled).
• Errors surfaced if API fails.

Test Checklist

Unit

• Space batch collects correct room IDs (reuse spaceUnread helpers).
• Unread state refreshes after batch mark.

Manuell

• Mark single room read from sidebar without selecting it.
• Mark space read clears rail badge + all channel dots in space.

Notes

• Related: Feature: Mention-priority unread notifications (@username) #35 unread indicators (done).

Epic

Sub-issue of #3 (Phase 3).