feat(messages): add support for message editing, deletion, and reactions

[sanity]

Problem

Users cannot edit or delete their own messages, nor can they react to messages with emoji. This limits the expressiveness and usability of the chat experience. (Issue #67)

Approach

Implements message actions (edit, delete, reactions) as special message types that flow through the normal message timeline:

1. Actions are messages: New RoomMessageBody variants (Edit, Delete, Reaction, RemoveReaction) are stored as signed messages in the timeline
2. Computed state: MessagesV1::actions_state tracks the effective state (edited content, deleted messages, reactions) - rebuilt from action messages on each delta
3. Authorization: Only the original message author can edit/delete their own messages. Anyone can add reactions, and users can only remove their own reactions
4. Display: UI shows "(edited)" indicator on edited messages and displays reaction badges below messages

This approach preserves the append-only, signed message model while enabling interactive features.

Why this design over alternatives?

• Actions as messages allows them to sync naturally through the existing delta mechanism
• Computed state with #[serde(skip)] avoids signature invalidation issues - the signed message content is never mutated
• Authorization in rebuild_actions_state() means invalid actions (e.g., editing someone else's message) are simply ignored, not rejected

Testing

New unit tests added (7 tests):

• test_edit_action - Verify edit applies and edited flag is set
• test_edit_by_non_author_ignored - Verify unauthorized edits don't apply
• test_delete_action - Verify deleted messages are marked and filtered from display
• test_reaction_action - Verify reactions from multiple users aggregate correctly
• test_remove_reaction_action - Verify users can remove their own reactions
• test_action_on_deleted_message_ignored - Verify actions on deleted targets are no-op
• test_display_messages_filters_actions - Verify action messages are not shown directly

Local validation:

• cargo test -p river-core - All 94 tests pass
• cargo clippy -p river-core - No errors (some pre-existing warnings)
• cargo fmt - All code formatted

What still needs testing:

• Full UI testing with multiple simulated users (requires dev server setup)
• Multi-peer sync testing to verify actions propagate correctly

Breaking Change

This adds new RoomMessageBody enum variants, which will cause deserialization errors for older clients. This is acceptable during alpha. Before production, consider adding #[serde(other)] fallback variant.

Critical Files

File	Changes
common/src/room_state/message.rs	+569 lines: New variants, computed state, delta processing, helper methods, tests
ui/src/components/conversation.rs	+109 lines: Display edited indicator and reactions
ui/src/components/conversation/message_actions.rs	New file: Context menu/emoji picker component (basic structure)
ui/src/components/app/notifications.rs	Handle new message types in notification previews

Future Work (not in this PR)

• Wire up context menu to actually send Edit/Delete/Reaction messages
• Add emoji picker UI interaction
• CLI support for edit/delete/react commands

Closes #67

[AI-assisted - Claude]