Only mark Inbox items read on explicit action

## Problem

In the Desktop Inbox, opening or selecting an unread row immediately marks the item read. This makes browsing destructive to triage state: a user cannot preview several unread items without changing the queue, and an item can disappear from **Show unread only** merely because it was selected.

Current code couples the two actions in the Inbox selection handler: after selecting the item, it calls `markItemRead(itemId)`.

- Selection handler: https://github.com/block/buzz/blob/63496cc1d4c6f1b7c613801bdcc694169dcf391a/desktop/src/features/home/ui/HomeView.tsx#L673-L687
- Explicit row-level **Mark as read** action already exists: https://github.com/block/buzz/blob/63496cc1d4c6f1b7c613801bdcc694169dcf391a/desktop/src/features/home/ui/InboxListPane.tsx#L421-L435
- Read action advances the shared channel/thread/message read markers: https://github.com/block/buzz/blob/63496cc1d4c6f1b7c613801bdcc694169dcf391a/desktop/src/features/home/useHomeInboxReadState.ts#L181-L227

## Steps to reproduce

1. Open the Desktop Inbox with one or more unread items.
2. Enable **Show unread only**.
3. Select an unread row to preview its conversation in the detail pane.
4. Observe that selection marks it read; moving to other items drains or changes the unread queue.

## Expected behavior

Use a Slack-style inbox triage model: selection previews an item, while read/completed state changes only through an explicit checkoff or **Mark as read** action. Selected and unread should remain separate states.

This request does not prescribe a storage or protocol implementation. The existing explicit read action and shared NIP-RS markers are relevant behavior to preserve.

## Acceptance criteria

- Opening or previewing an unread Inbox item does not mark it read.
- Read state changes only through an explicit checkoff or mark-read action.
- Users can move through multiple unread items without losing their queue.
- Existing manual **Mark unread** behavior remains coherent with explicit completion.
- Keyboard and accessibility behavior are defined: preview/select and mark-read are distinct operable actions with clear focus and screen-reader labels.
- An explicit read-state change persists correctly across refresh and devices.
- Selected styling is distinguishable from unread styling.
- In **Show unread only**, previewing the selected row does not remove it from the list; explicit completion follows a predictable next-item behavior.

## Related work reviewed

- #102 broadly requested read/done handling and was closed after the original Inbox work; it did not establish explicit-only semantics.
- #1114 added Inbox read/unread handling.
- #1116 described marking unread rows read when opened; it is closed and not a request to decouple selection.
- #2045 merged the current conversation-oriented Inbox behavior.
- #3669 and its open PRs concern persistence of the **Show unread only** preference, not when an item becomes read.
- #3365 concerns scroll targeting after read-state changes, not explicit checkoff semantics.

GitHub Discussions are disabled for this repository, so there was no discussion thread to match.