[FEATURE]: Add desktop wide mode to use full available chat width #29408

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Please add a Desktop/Web “Wide Mode” setting so the chat/session content can use the full available window width.
Right now the desktop app caps the session/chat content width and leaves large empty margins on wide windows. This is frustrating enough that I keep using the TUI instead of the desktop app. The narrow centered column is a major blocker for switching to desktop.

Why this matters:

• Wide monitors waste significant screen space.
• Code blocks, diffs, and tool output wrap sooner than necessary.
• The terminal/TUI experience feels much better because it uses available width.
• Desktop should let users decide whether they want a readable centered column or full-width workspace.

Expected behavior:

• Add a setting like “Wide Mode” in Desktop/Web settings.
• Keep current centered layout as default if preferred.
• When enabled, session timeline, sticky header, and composer should align and use full available session panel width.
• Optionally expose config via opencode.json if app settings are not preferred.

Prior related discussions/PRs:

• feat(app): add wide mode setting to use full window width for chat #14835
• Desktop/Web app: content area has narrow max-width, doesn't use full window width #16567
• Desktop UI should prioritize desktop workflows and screen real estate #15735