app: session content locked to 744px on wide windows - no way to use the remaining width #56953

Description

On any window wider than 768px, the session transcript, the composer and the page container are clamped to a fixed 744px column and centered, leaving large empty margins on both sides. There is no setting, no command and no drag handle to change it, so on a large display most of the horizontal space is unusable.

This is not desktop-specific: the width is derived from a media query, so the web UI behaves identically as soon as the window is wider than 768px.

Steps to reproduce

1. Open OpenCode Desktop and maximize the window (or open the web UI and widen the window past 768px).
2. Open or create any session.
3. Observe that the message column, the composer and the session header stay in a 744px column centered in the window, with the remaining width left empty on both sides.
4. Look for any way to widen it — there is none in Settings, in the command palette, or by dragging.

Environment

• UI: OpenCode Desktop 2.0.32 (Windows 11)
• Also reproducible on the web UI at any window width > 768px
• Verified against current dev source (ba06ff9e0b)

Root cause (verified on current dev)

The clamp is driven by a media query rather than by any user preference:

// packages/app/src/session/model.ts:29
const isDesktop = createMediaQuery("(min-width: 768px)")

// packages/app/src/session/screen-layout.ts:106
centered: createMemo(() => session.isDesktop()),

centered then switches a Tailwind max-w-session utility on:

// packages/app/src/session/timeline/message-timeline.tsx:406
"md:max-w-session md:mx-auto": props.centered,

/* packages/ui/src/styles/tailwind/index.css:53 */
--container-session: 744px;

The constant is consumed in seven places (line numbers as of ba06ff9e0b):

Because centered is derived purely from the viewport, there is no code path a user or config can influence.

Impact

On a 2560px-wide window roughly 1070px per side is unused. For long transcripts this means more scrolling for the same content, and code or diff blocks wrap earlier than they need to.

I understand 744px is a deliberate comfortable-reading width, so this issue is not asking to change the default.

Suggested direction

Keep --container-session: 744px as the default, but let it be overridden per user:

1. persist a width preference (e.g. localStorage, consistent with other UI preferences) and set --container-session from it, so all seven consumers follow automatically;
2. add a drag handle on the session column edges (double-click to reset to the default);
3. surface the current width plus a "reset to default" affordance in Settings, so the value is discoverable rather than only reachable by dragging;
4. keep narrow viewports (< 768px) on the existing full-width behaviour.

The new-session page has its own hardcoded 880px in packages/app/src/new-session/layout.ts:2; leaving it out of scope for now, but flagging it since it has the same limitation.

Screenshots

Not attached — the reproduction above and the root-cause analysis are self-contained. A before/after pair will be attached to the PR.