merge-prep: Polish agentic chat + Vertex (supersedes #490)

[spashii]

Supersedes #490 by @dtrn2048 — same commits, plus a merge commit that resolves conflicts against current main so it can be merged without a dirty state.

What changed vs #490

Nothing in @dtrn2048's commits. The tip of this branch is a merge commit (merge: bring main into feat/agentic-chat-main) that resolves:

• echo/frontend/src/components/chat/ChatHistoryMessage.tsx — only real code conflict. Union of imports: ActionIcon (from main) + IconArrowUpRight/IconNotes (from feature). Both are referenced in the file body.
• echo/frontend/src/locales/*.po — 12 mechanical conflicts across 6 languages (new msgids on both sides + shifted source line numbers). Took union, then re-ran lingui extract to canonicalize #: paths.
• echo/frontend/src/locales/*.ts — regenerated with lingui compile --typescript.

tsc --noEmit passes locally.

Why a fresh PR

PR #490 has been open since 2026-03-19; main is +27 commits ahead and the original PR shows dirty. Rather than force-push to @dtrn2048's branch, this is a fresh branch that supersedes it. @dtrn2048's authorship on the original commits is preserved.

Footprint vs the workspaces epic

Overlap with the workspaces branch: 9 files (7 locales). Real overlap on api.ts (+60 on workspaces) and agentic.py (+14). Bounded — should not block the workspaces rebase later.

Confidence

Medium-high. Conflict resolution is small and reviewable. CI will run lint/types/tests. The one judgment call worth a second look is the ChatHistoryMessage.tsx import union — sanity-check that both ActionIcon and the tabler icons should coexist in the merged version.

What I did not do

• Did not merge. Human call.
• Did not run anything Vertex/GCP-side. Schema/config changes are in @dtrn2048's PR description; deploy-time concerns unchanged.

Original PR: #490

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