feat(desktop): polish agent-session transcript UI atop declared-ownership model

Replay of PR #1061 (tho/activity-ui-polish) onto fresh main: lands the
20 UI-polish commits as one net diff, reconciled with #1229's merged
declared-ownership model (viewerIsOwner = isCurrentUserOwner || isOwner)
and #1089's content-visibility-auto virtualization.

Notable reconciles:
- markdown.tsx: ported the compact/tight variant system into main's
  newer lightbox/spoiler markdown component (rather than overwriting it),
  layering variant density/leading overrides after the base owl-spacing
  so tailwind-merge wins. Dropped the branch's hardcoded text-[15px] in
  favor of main's rem-token text-sm base (post-#1052 zoom-safe scale).
- agentSessionTranscript.ts: pass TranscriptItemContext (not channelId).
- managed_agents: thread avatar_url through ManagedAgentSummary so the
  transcript renders the assistant-bubble avatar from the pinned record
  snapshot; bumped runtime.rs size override 2001 -> 2002 for the +1 line.

Observer-seed screenshots intentionally excluded (separate follow-up).

Co-authored-by: Taylor Ho <taylorkmho@gmail.com>
Signed-off-by: Taylor Ho <taylorkmho@gmail.com>

Author: 2 people

desktop/scripts/check-file-sizes.mjs

@@ -46,8 +46,10 @@ const overrides = new Map([
   ["src-tauri/src/managed_agents/nest.rs", 1450],
   // harness-persona-sync: persona-runtime resolution threaded into the spawn
   // path here. Load-bearing feature growth; queued to split in the resolver
-  // unify refactor followup.
-  ["src-tauri/src/managed_agents/runtime.rs", 2001],
+  // unify refactor followup. +1 line from the activity-ui-polish replay:
+  // build_managed_agent_summary now threads avatar_url so the transcript UI can
+  // render the assistant-bubble avatar from the pinned record snapshot.
+  ["src-tauri/src/managed_agents/runtime.rs", 2002],
   ["src-tauri/src/managed_agents/personas.rs", 1080],
   // Phase-2 inbound reconcile + review-fix cycle: reconcile_inbound_persona_event
   // dispatches 30175/30176/30177 inbound plus kind:5 tombstone consume

desktop/src-tauri/src/managed_agents/runtime.rs

@@ -1405,6 +1405,7 @@ pub fn build_managed_agent_summary(
         max_turn_duration_seconds: record.max_turn_duration_seconds,
         parallelism: record.parallelism,
         system_prompt: record.system_prompt.clone(),
+        avatar_url: record.avatar_url.clone(),
         model: record.model.clone(),
         provider: record.provider.clone(),
         persona_out_of_date,

desktop/src-tauri/src/managed_agents/types.rs

@@ -268,6 +268,7 @@ pub struct ManagedAgentSummary {
     pub max_turn_duration_seconds: Option<u64>,
     pub parallelism: u32,
     pub system_prompt: Option<String>,
+    pub avatar_url: Option<String>,
     pub model: Option<String>,
     /// LLM inference provider, from the agent's pinned record snapshot.
     pub provider: Option<String>,