refactor(renderer): route all Phosphor icon imports through ui/icons barrel

Create src/renderer/components/ui/icons.ts as the single re-export point
for every Phosphor icon the renderer touches, then rewrite the 95 files
that previously imported directly from "@phosphor-icons/react" to pull
from the relative-path-to-barrel instead. The TitleBar commit earlier
in this series already used ui/icons; this commit extends the pattern
to the rest of the renderer.

Benefits now wired in:
  - one location where bundle tree-shaking can be reasoned about,
  - typos like Briefcase vs BriefCase show up against a finite list,
  - re-theming or swapping icon sets is a one-file edit,
  - a lint rule that flags raw @phosphor-icons/react imports can be
    added later without mass-rewriting call sites.

The barrel also re-exports the `Icon` type so call sites that type
icon props against the Phosphor component type work unchanged.

Test count stays at 363 passing; typecheck stays at 156 errors.

src/renderer/components/communication/MessageItem.tsx

@@ -8,7 +8,7 @@ import {
   WarningCircle,
   ArrowBendUpLeft,
   UserCircle,
-} from "@phosphor-icons/react"
+} from "../ui/icons"
 import type { AgentMessageRecord, AgentRecord, CommentRecord, DepartmentId } from "@shared/types"
 import { getAgentInitials, formatMessageTime } from "../../lib/formatters"
 import { safeJson } from "../../lib/safe-json"