feat(chat): sent images, line diffs, compaction marker, h4-h6 - #73

The row takes a line above the message box on every pane; it now waits for
the ⚡ button, and the choice is remembered per device as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Transcripts already record what each request filled:
- Codex: token_count events with last_token_usage and model_context_window
- Claude Code: assistant usage, input + cache creation + cache read,
  subagent (sidechain) entries skipped
- omp/omo/gjc: message usage, input + cacheRead + cacheWrite

ConversationMetadata gains an optional `context { used, window }`. The
window is only what the transcript leaves no doubt about: Codex states it,
and a Claude request past 200k can only have run in the 1M window (kept
through a compaction). Otherwise the line shows the tokens used, never a
guessed percentage.

The composer shows `74% left` or `68k used`, red at 20% or less, with the
counts on hover. PaneTerminal's metadata dedupe compared only model and
effort, so it now compares the context too, or the number would never move.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

…, a loading state

From comparing the chat with chatmux's:

- A failed tool call looked like a success inside "Worked for…". Tool parts
  gain `error`: Claude's tool_result is_error and omp's toolResult isError,
  and for Codex, which records no flag, the output's own verdict
  (exit code, "Script failed", "apply_patch verification failed"; a
  completed script is judged as a whole). The row reads "failed" in red,
  its output is headed Error, and the block header counts failures.
- A file path in a tool row opens the FileViewer, like paths in prose.
- Codex's apply_patch, bare or as the string an exec script passes to
  tools.apply_patch, renders as a per-file diff and its row is summed up
  by the files it touches (shared/patch.ts, used by server and client).
- The jump-to-latest button showed only for new messages; it now shows
  whenever the reader is scrolled up (a round arrow), and stays the "New
  messages" pill when there are some.
- The chat said "No conversation yet" until the first answer came back;
  it now says it is loading.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

More from comparing with chatmux's chat:

- Images the user sent: Claude transcripts keep pasted images as base64
  in the user entry. The parser now emits an image part naming it
  (`<entry uuid>:<block index>`, png/jpeg/gif/webp only) instead of the
  data, and GET /api/pane/conversation/image decodes it on request
  (immutable cache), so a 2 s poll never carries megabytes. Images this
  app attaches appear as `@…png` mentions and show through /api/fs/file.
  Thumbnails sit above the user bubble; a tap opens them.
- Edit/MultiEdit render as one line diff (src/lib/diff.ts, an LCS that
  falls back to removed-then-added past 250k line pairs).
- A compaction's summary, which was dropped, is a divider that opens to
  it.
- Markdown headings #### to ###### render as h4-h6.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>