feat(walkthrough): write walkthroughs in the reader's language

A guided explanation is only useful in a language the reader reads, so the
panel header gets a language picker alongside the model one, defaulting to
the interface language. Like the model, it is request state rather than a
setting: the language travels with the read and the generation, and the one
a walkthrough was written in is stored with it, so reopening a review
describes what is there instead of what a fresh one would be.

Only prose is translated. Hunk aliases resolve back to hunk ids and
icon/importance are validated against fixed English values, so a translated
one would be dropped by the normalizer — silently losing an anchor or a
style. Identifiers and paths stay as they appear in the code.

The language is part of the cache key, and a read now asks the cache for the
exact request it was given before falling back to the pointer. Without that
the panel answered a request to switch languages with the text it already
had, leaving the other language unused in the cache.

Alongside it:

- The answer budget is derived from the resolved model instead of a flat 24k.
  That number was the same for a 64k-context model and for one that admits to
  384k output tokens, and on the latter it was the only reason generation
  failed: the model spent the whole allowance reasoning and returned nothing.
  It is now min(96k, max(24k, a quarter of the context)) capped by the
  catalog's output limit, decided once so the input reserve and the request
  cannot drift apart.
- A read no longer offers Cancel. It is a few hundred milliseconds of git with
  nothing to cancel, and the button flickered on every model or language
  change. When the panel is showing a fallback, a banner names what is on
  screen versus what was asked for — only once the read has settled.
- The header keeps one 32px control height and drops its labels below 680px
  instead of squeezing them to two letters and an ellipsis.

Docs and module documentation updated in every locale.

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
-- **Walkthrough:** a new guided walkthrough reorders a diff into a sequence of stops — the model groups related changes, explains what each one does, and orders them so each builds on the last. Start one from the Changes and pull-request views for uncommitted work, a branch against its base, or a pull request; nothing runs on its own.
+- **Walkthrough:** a new guided walkthrough reorders a diff into a sequence of stops — the model groups related changes, explains what each one does, and orders them so each builds on the last. Start one from the Changes and pull-request views for uncommitted work, a branch against its base, or a pull request; nothing runs on its own. Walkthroughs are written in your interface language by default, and the panel can generate one in any other supported language.
 - **Mobile/Tablet:** reworked the tablet and foldable layout around the phone's navigation — a persistent resizable sessions sidebar on the left, the workspace (Changes, Files, Terminal, Notes, MCP) as a resizable right sidebar, and app pages like settings and instances shown as centered dialogs. An open diff, edited file, or attached terminal now survives rotation.
 - Performance: fixed Bun dependency chunking so the web app no longer downloads a single 18.5 MB vendor bundle at startup; heavy syntax highlighting, screenshot, diagram, editor, and image-conversion libraries now load only when needed (thanks to @makeittech).
 - Mobile/Android: pairing QR codes can now be scanned on devices without Google Play Services; the camera closes as soon as a code is recognized, followed by a connection-in-progress screen.