W21 Phase 2: drag-selectable chat messages (per-message)

Add `SelectableRichText`, a custom iced widget that mirrors `rich_text`
(linkified spans + A13 link clicks) and adds per-message drag selection,
Ctrl/Cmd+C copy, and Ctrl/Cmd+A select-all. Swap it in for the chat body
element; the chat row and attachment layout are unchanged.

Selection offsets are paragraph-global byte offsets (matching cosmic-text's
hit_test), which equals a single global range because sanitize_chat keeps
every message on one logical line. Pure seam `selected_substring` /
`select_all` is unit-tested incl. unicode/emoji byte boundaries. Highlight
quads are computed from public Paragraph primitives, falling back to a
whole-message span_bounds union if sub-range rects can't be derived.

Only one message holds a selection at a time: each widget clears its own
selection on a left-press that lands outside its bounds. Right-click menu
(Part B) intentionally deferred — native Ctrl/Cmd+C/A is the path.

Implemented by Codex on branch, reviewed/committed by Claude.
464 lib tests, clippy --all-targets clean, release green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -12,11 +12,12 @@ use crate::hotkeys::{format_binding, HotkeyAction, HotkeyContext, KeyBinding};
 use crate::presence::PresenceMode;
 use crate::theme::{AppTheme, Palette};
 use crate::widget::context_input::{context_input, locked_value};
+use crate::widget::selectable_text::selectable_rich_text;
 
 use iced::widget::{
     container, column, row, text, button, scrollable, slider, checkbox, pick_list,
     radio, tooltip, progress_bar, canvas, Canvas, Column, stack, mouse_area,
-    rich_text, span, responsive,
+    span, responsive,
 };
 use iced::widget::text_input;
 use iced::widget::canvas::{Action, Frame, Geometry, Path, Program};
@@ -4670,8 +4671,9 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                         }
                     })
                     .collect();
-                let body = rich_text(spans)
+                let body = selectable_rich_text(spans)
                     .on_link_click(AppMessage::OpenUrl)
+                    .selection_color(color_blue)
                     .width(iced::Length::Fill);
                 // Small avatar keyed on the sender's id (falls back to name); the
                 // " (You)" suffix on our own echoes is stripped for clean initials.