feat: display message reactions

[guitaripod]

What

Closes #2 (reactions portion) / revives intent of #27.

Reactions were already parsed from the REST message payload (parse_reactions in client.rs) and the gateway codec already decoded MESSAGE_REACTION_ADD, MESSAGE_REACTION_REMOVE, and MESSAGE_REACTION_REMOVE_ALL. The app subscribed to the reaction intent but the three dispatch events only emitted debug! logs, and the message pane never drew the reactions that did arrive via REST. This PR closes that gap.

Changes

• domain/entities/message.rs — Message::add_reaction / remove_reaction / clear_reactions encapsulate list mutation (increment/decrement with me tracking, drop entries at zero). ReactionEmoji::display() renders unicode emoji verbatim and custom guild emoji as :name:.
• presentation/widgets/message_pane.rs — new format_reaction_lines builds indented chip rows ({emoji} {count}) that wrap on content_width, UiMessage caches them, layout_message and calculate_message_height fold them into the existing O(1) geometry, and render_ui_message paints them after embeds. MessagePaneStyle gains reaction_me_style (theme accent + bold) and reaction_other_style (dimmed). MessagePaneData::apply_reaction_{add,remove,remove_all} mutate the cached Arc<Message> via Arc::make_mut and invalidate layout for that row only.
• presentation/ui/chat_screen.rs / app.rs — wires the three gateway dispatch events through to the pane, resolving is_me against current_user_id.

No new dependencies. No new allocations on the hot render path beyond the cached chip lines. The reaction intent was already enabled via GatewayIntents::with_reactions().

Visual proof

Snapshot test that renders the reactions through a real ratatui::buffer::Buffer (reaction_lines_buffer_snapshot, run with -- --nocapture):

--- reactions (width 40) ---
      ❤ 3  👍  1  :wave: 2               
--- end ---
--- reactions (width 16, wrapped) ---
      ❤ 3  👍  1 
      :wave: 2  
--- end ---

The first snapshot shows unicode emoji (❤, 👍) and a custom guild emoji (:wave:) rendered inline with their counts at the CONTENT_INDENT column. The second shows the same reactions forced to wrap at width 16 — chips never split mid-chip, and each wrapped row re-applies the indent so the block stays visually aligned with the message body. In a real terminal session, ❤ 3 would be drawn in the theme accent + bold (it has me: true); 👍 1 and :wave: 2 in dim gray.

Tests

10 new unit tests, all green:

domain::entities::message::reaction_tests::add_reaction_inserts_new_entry ... ok
domain::entities::message::reaction_tests::add_reaction_increments_existing_and_sets_me ... ok
domain::entities::message::reaction_tests::remove_reaction_decrements_and_drops_at_zero ... ok
domain::entities::message::reaction_tests::clear_reactions_empties_list ... ok
domain::entities::message::reaction_tests::display_formats_custom_and_unicode ... ok
presentation::widgets::message_pane::tests::reaction_lines_empty_when_no_reactions ... ok
presentation::widgets::message_pane::tests::reaction_lines_wrap_when_exceeding_width ... ok
presentation::widgets::message_pane::tests::reaction_lines_apply_me_style_only_to_self_chip ... ok
presentation::widgets::message_pane::tests::reaction_lines_buffer_snapshot ... ok
infrastructure::discord::client::tests::test_message_response_parsing_with_reactions ... ok

Full suite: test result: ok. 275 passed; 0 failed; 1 ignored. cargo clippy --all-targets introduces no new warnings.

Not in scope

• Adding/removing one's own reactions (write path, reaction picker UI, keybind). Happy to do this as a follow-up; this PR is strictly the read/display side so the feedback loop stays short.
• Emoji image rendering for custom guild emoji. The terminal can't draw those images, so :name: is the canonical representation.

[linuxmobile]

I don't have enough time to review the PR right now, but I'll take a look as soon as I have a chance. I have a lot of work to do with the repository (issues and such).