Show a replayed session's images instead of dropping them

An imported session showed no screenshots. `text_of` kept only `text`
blocks, so every image in the replayed tail was silently discarded -- while
the *live* translator has always saved them into the session's `files/` and
referenced them. Two readings of the same records, and the one used for
history was the lesser.

`save_image` moves out of `Translator` to a free function both paths call,
since the naming scheme for that directory should exist once. `events_from`
now takes the session directory to write into, which means the conversion
has to happen where that directory exists -- so `Seed` carries the raw
JSONL and `launch` turns it into events, rather than `routes` doing it
before the session is created.

Costs nothing in tokens, which is the point worth recording: this writes
into ai-app's own session directory and the phone fetches a reference only
when it draws one. Nothing here is ever written to the CLI's stdin -- it
reads its own session file, and the only things this app sends it are typed
messages, control requests and `/compact`.

Verified against the 133 MB session behind the 2026-08-29 incident: 45
images in the replayed tail, written as real PNGs and served over the files
route, with the transcript itself staying at 756 KB of references.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VETa8afmpWaYezLCqJhDB8

AGENTS.md

@@ -41,6 +41,11 @@ repo is in PLAN.md's "Backend layout" section.
   `POST /setups`. Only the tail is replayed (`REPLAY_LINES`) because these
   files reach tens of megabytes and the CLI reads the real one itself; what
   crosses the tunnel is what a person reads, not what the model is given.
+  Images in the replayed tail are written into the session's `files/` by
+  the same function the live translator uses, and referenced -- so a
+  screenshot looks the same whether it was watched happening or replayed
+  afterwards, the phone fetches the bytes only when it draws one, and none
+  of it goes anywhere near the CLI, which reads its own file.
   An imported session then **keeps itself level with that file**, so work
   done at a terminal appears without anyone pressing anything. `--resume`
   appends to the same transcript rather than forking — measured, but